Observing pulsars and fast transients with LOFAR

Abstract

Low frequency radio waves, while challenging to observe, are a rich source of
information about pulsars. The LOw Frequency ARray (LOFAR) is a new radio
interferometer operating in the lowest 4 octaves of the ionospheric "radio
window": 10-240MHz, that will greatly facilitate observing pulsars at low radio
frequencies. Through the huge collecting area, long baselines, and flexible
digital hardware, it is expected that LOFAR will revolutionize radio astronomy
at the lowest frequencies visible from Earth. LOFAR is a next-generation radio
telescope and a pathfinder to the Square Kilometre Array (SKA), in that it
incorporates advanced multi-beaming techniques between thousands of individual
elements. We discuss the motivation for low-frequency pulsar observations in
general and the potential of LOFAR in addressing these science goals. We
present LOFAR as it is designed to perform high-time-resolution observations of
pulsars and other fast transients, and outline the various relevant observing
modes and data reduction pipelines that are already or will soon be implemented
to facilitate these observations. A number of results obtained from
commissioning observations are presented to demonstrate the exciting potential
of the telescope. This paper outlines the case for low frequency pulsar
observations and is also intended to serve as a reference for upcoming
pulsar/fast transient science papers with LOFAR.

Full text

Astronomy & Astrophysics manuscript no. lofarPSR c
ESO 2011
April 11, 2011
Observing pulsars and fast transients with LOFAR
B. W. Stappers1, J. W. T. Hessels2,3A. Alexov3, K. Anderson3, T. Coenen3, T. Hassall1, A. Karastergiou4,
V. I. Kondratiev2, M. Kramer5,1, J. van Leeuwen2,3, J. D. Mol2, A. Noutsos5, J. W. Romein2,
P. Weltevrede1, R. Fender6, R. A. M. J. Wijers3, L. B¨ahren3, M. E. Bell6, J. Broderick6, E. J. Daw8,
V. S. Dhillon8, J. Eisl¨offel19, H. Falcke12,2, J. Griessmeier2,22, C. Law24,3, S. Markoff3
J. C. A. Miller-Jones13,3, B. Scheers3, H. Spreeuw3, J. Swinbank3, S. ter Veen12 M. W. Wise2,3,
O. Wucknitz17, P. Zarka16, J. Anderson5, A. Asgekar2, I. M. Avruch2,10, R. Beck5, P. Bennema2,
M. J. Bentum2, P. Best15, J. Bregman2, M. Brentjens2, R. H. van de Brink2, P. C. Broekema2,
W. N. Brouw10, M. Br¨uggen21, A. G. de Bruyn2,10, H. R. Butcher2,26, B. Ciardi7, J. Conway11, R.-J.
Dettmar20, A. van Duin2, J. van Enst2, M. Garrett2,9, M. Gerbers2, T. Grit2, A. Gunst2, M. P. van
Haarlem2, J. P. Hamaker2G. Heald2, M. Hoeft19, H. Holties2, A. Horneffer5,12, L. V. E. Koopmans10,
G. Kuper2, M. Loose2, P. Maat2, D. McKay-Bukowski14, J. P. McKean2, G. Miley9, R. Morganti2,10,
R. Nijboer2, J. E. Noordam2, M. Norden2, H. Olofsson11, M. Pandey-Pommier9,25, A. Polatidis2, W. Reich5,
H. R¨ottgering9, A. Schoenmakers2, J. Sluman2, O. Smirnov2, M. Steinmetz18, C. G. M. Sterks23,
M. Tagger22, Y. Tang2, R. Vermeulen2, N. Vermaas2, C. Vogt2, M. de Vos2, S. J. Wijnholds2, S. Yatawatta10,
and A. Zensus5
1Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester
M13 9PL,UK e-mail: Ben.Stappers@manchester.ac.uk
2Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, The Netherlands
3Astronomical Institute ’Anton Pannekoek’, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The
Netherlands
4Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH
5Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, 53121 Bonn, Germany
6School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
7Max Planck Institute for Astrophysics, Karl Schwarzschild Str. 1, 85741 Garching, Germany
8Department of Physics & Astronomy, Hicks Building, Hounsfield Road, Sheffield S3 7RH, United Kingdom
9Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
10 Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
11 Onsala Space Observatory, Dept. of Earth and Space Sciences, Chalmers University of Technology, SE-43992 Onsala,
Sweden
12 Department of Astrophysics/IMAPP, Radboud University Nijmegen, P.O. Box 9010, 6500 GL Nijmegen, The
Netherlands
13 International Centre for Radio Astronomy Research - Curtin University, GPO Box U1987, Perth, WA 6845, Australia
14 STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK
15 Institute for Astronomy, University of Edinburgh, Royal Observatory of Edinburgh, Blackford Hill, Edinburgh EH9
3HJ, UK
16 LESIA, UMR CNRS 8109, Observatoire de Paris, 92195 Meudon, France
17 Argelander-Institut f¨ur Astronomie, University of Bonn, Auf dem H¨ugel 71, 53121, Bonn, Germany
18 Leibniz-Institut fr Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
19 Th¨uringer Landessternwarte, Sternwarte 5, D-07778 Tautenburg, Germany
20 Astronomisches Institut der Ruhr-Universit¨at Bochum, Universitaetsstrasse 150, 44780 Bochum, Germany
21 Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany
22 Laboratoire de Physique et Chimie de lEnvironnement et de lEspace, CNRS/Universit dOrlans, France
23 Center for Information Technology (CIT), University of Groningen, The Netherlands
24 Radio Astronomy Lab, UC Berkeley, CA, USA
25 Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 av Charles Andr´e, 69561 Saint Genis Laval
Cedex, France
26 Mt Stromlo Observatory, Research School of Astronomy and Astrophysics, Australian National University, Weston,
A.C.T. 2611, Australia
ABSTRACT
Low frequency radio waves, while challenging to observe, are a rich source of information about pulsars. The LOw
Frequency ARray (LOFAR) is a new radio interferometer operating in the lowest 4 octaves of the ionospheric ”radio
window”: 10-240MHz, that will greatly facilitate observing pulsars at low radio frequencies. Through the huge collecting
area, long baselines, and flexible digital hardware, it is expected that LOFAR will revolutionize radio astronomy at
the lowest frequencies visible from Earth. LOFAR is a next-generation radio telescope and a pathfinder to the Square
Kilometre Array (SKA), in that it incorporates advanced multi-beaming techniques between thousands of individual
elements. We discuss the motivation for low-frequency pulsar observations in general and the potential of LOFAR in
addressing these science goals. We present LOFAR as it is designed to perform high-time-resolution observations of
pulsars and other fast transients, and outline the various relevant observing modes and data reduction pipelines that are
already or will soon be implemented to facilitate these observations. A number of results obtained from commissioning
observations are presented to demonstrate the exciting potential of the telescope. This paper outlines the case for low
frequency pulsar observations and is also intended to serve as a reference for upcoming pulsar/fast transient science
papers with LOFAR.
Key words. telescopes:LOFAR – pulsars:general – instrumentation:interferometric – methods:observational –
stars:neutron – ISM:general
1
arXiv:1104.1577v1 [astro-ph.IM] 8 Apr 2011

1. Introduction
Pulsars are rapidly rotating, highly magnetised neutron
stars that were first identified via pulsed radio emission at
the very low radio observing frequency of 81 MHz (Hewish
et al., 1968). They have subsequently been shown to emit
pulsations across the electromagnetic spectrum, at frequen-
cies ranging from 17 MHz to above 87 GHz (e.g. Bruk &
Ustimenko 1976, 1977; Morris et al. 1997) in the radio and
at optical, X-ray and γ-ray wavelengths (see Thompson
2000 and references therein), although the vast majority
are seen to emit only at radio wavelengths. These pulsa-
tions provide invaluable insights into the nature of neutron
star physics, and most neutron stars would be otherwise
undetectable with current telescopes. Though radio pul-
sars form over 85% of the known neutron star population,
they are generally very weak radio sources with pulsed flux
densities ranging from 0.0001 to 5 Jy with a median of
0.01 Jy at a frequency of 400 MHz. The pulsed flux density
at radio wavelengths exhibits a steep spectrum (S∝να;
−4< α < 0; αmean =−1.8, (Maron et al., 2000) that of-
ten peaks and turns over at frequencies between 100 and
200 MHz (Kuzmin et al., 1978; Slee et al., 1986; Malofeev
et al., 1994).
After their discovery, a lot of the early work on pulsars
(e.g. Cole 1969; Staelin & Reifenstein 1968; Rankin et al.
1970) continued at low radio frequencies (defined here as
<300 MHz). However, despite the fact that most pulsars
are intrinsically brightest in this frequency range, since then
the vast majority of pulsars have been discovered and stud-
ied at frequencies in the range 300 −2000 MHz; much of
our knowledge of the properties of the radio emission mech-
anism stems from studies at these frequencies and above.
There are three main reasons for this (see Sect. 3): the dele-
terious effects of the interstellar medium (ISM) on pulsed
signals; the effective background sky temperature of the
Galactic synchrotron emission; and ionospheric effects. All
three of these effects have steep power law dependencies
on frequency and therefore become worse towards lower
frequencies. Combined with the generally steep spectra of
pulsars, these effects conspire to make observing frequen-
cies of ∼300−2000 MHz the range of choice for most pulsar
studies and searches.
However, despite these challenges, there are many rea-
sons why it is important and interesting to observe pul-
sars in a significantly lower frequency regime than now
commonly used; these are discussed in detail in Sect.
4. In recent years some excellent studies have contin-
ued at frequencies between 20−110 MHz mainly using
the Pushchino, Gauribidanur and UTR-2 telescopes (e.g.
Malov & Malofeev 2010; Malofeev et al. 2000; Asgekar &
Deshpande 2005; Popov et al. 2006b; Ulyanov et al. 2006).
These studies have begun to map, e.g., the low-frequency
spectra, pulse morphologies, and pulse energy distributions
of pulsars, but have in some cases been limited by the avail-
able bandwidths and/or polarisation and tracking capabil-
ities of these telescopes (see Sect. 2).
The Low Frequency Array (LOFAR) was designed and
constructed by ASTRON, the Netherlands Institute for
Radio Astronomy, and has facilities in several countries,
that are owned by various parties (each with their own
funding sources), and that are collectively operated by the
International LOFAR Telescope (ILT) foundation under a
joint scientific policy. LOFAR provides a great leap forward
in low-frequency radio observations by providing large frac-
tional bandwidths and sophisticated multi-beaming capa-
bilities. In this paper we present the LOFAR telescope as it
will be used for pulsar and other high-time-resolution beam-
formed observations; this will serve as a reference for future
science papers that use these LOFAR modes. We also de-
scribe the varied pulsar and fast transient science LOFAR
will enable and present commissioning results showing how
that potential is already being realised. LOFAR is well
suited for the study of known sources, and its huge field of
view (FoV) makes it a powerful survey telescope for find-
ing new pulsars and other “fast-transients”. In Sect. 2 we
present the basic design parameters of the LOFAR tele-
scope. The challenges associated with observing at low ra-
dio frequencies and how they can be mitigated with LOFAR
will be discussed in Sect. 3. A detailed description of the
science that will be possible with LOFAR is presented in
Sect. 4. The flexible nature of LOFAR means that there
are many possible observing modes; these are introduced in
Sect. 5. In Sect. 6 we discuss the different pulsar pipelines
that are being implemented. Commissioning results, which
demonstrate that LOFAR is already performing pulsar and
fast transients observations of high quality, are presented in
Sect. 7. We summarise the potential of LOFAR for future
pulsar observations in Sect. 8.
2. LOFAR
Instrumentation in radio astronomy is undergoing a revo-
lution that will exploit massive computing, clever antenna
design, and digital signal processing to greatly increase the
instantaneous FoV and bandwidth of observations. This
work is part of the international effort to create the “Square
Kilometre Array” (Carilli & Rawlings, 2004), a radio tele-
scope orders of magnitude better than its predecessors.
One of the first “next generation” radio telescopes to
implement these techniques is LOFAR, which operates in
the frequency range 10 −240 MHz. The large collecting
area of LOFAR is comprised of many thousands of dipole
antennas, hierarchically arranged in stations which come in
three different configurations (Table 1). These stations are
distributed in a sparse array with a denser core region near
Exloo, the Netherlands, extending out to remote stations in
the Netherlands and then on further to stations in France,
Germany, Sweden and the United Kingdom. There are a
total of 40 stations in the Netherlands and 8 international
stations, with the prospect of more to come. A schematic
diagram of some of the LOFAR stations in the inner core
of LOFAR −the “Superterp” as it is known −is shown in
Figure 1. As will be discussed in more detail later, pulsar
observations can utilise all of these stations to achieve a va-
riety of diverse science goals. Details of system architecture
and signal processing can be found in de Vos et al. (2009)
and a full description of LOFAR will soon be published (van
Haarlem et al. in prep.) we limit the discussion only to the
most important points related to pulsar observations.
LOFAR has two different types of antennas to cover
the frequency range 10–240 MHz. The low band anten-
nas, LBAs, cover the frequency range 10–90 MHz, although
they are optimised for frequencies above 30 MHz. The lower
limit of 10 MHz is defined by transmission of radio waves
through the Earth’s ionosphere. There are 48/96 active
LBA dipoles in each Dutch/international station (Table 1).
The high band antennas, HBAs, cover the frequency range

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
Fig. 1. Three successive zoom-outs showing the stations in the LOFAR core. The different scales of the hierarchically organised
HBA elements are highlighted and their respective beam sizes are shown. The large circular area marks the edge of the Superterp,
which contains the inner-most 6 stations (i.e. 12 HBA sub-stations: where there are 2 sub-stations, each of 24 tiles, in each HBA
core station); other core stations can be seen highlighted beyond the Superterp in the third panel. Left: a single HBA tile and
associated beam. Middle: A single HBA sub-station with three simultaneous station beams. Right: The 6 stations of the Superterp
plus 3 core stations in the background are highlighted. Four independent beams formed from the coherent combination of all 24
core HBA stations, most of which are outside this photo, are shown. For the LBA stations, a similar scheme applies except that
each LBA dipole can effectively see the whole sky. Fields of the relatively sparsely distributed LBA antennas are visible in between
the highlighted HBA stations in all three panels.
110–240 MHz, and consist of 16 folded dipoles grouped into
tiles of 4 ×4 dipoles each, which are phased together using
an analogue beamformer within the tile itself. There are
48/96 tiles in each Dutch/international station, with a sep-
aration into two sub-stations of 24 HBA tiles each in the
case of core stations1.
The received radio waves are sampled at either 160 or
200 MHz in one of three different Nyquist zones to access
the frequency ranges 0–100, 100–200 and 160–240 MHz.
There are filters in place to optionally remove frequen-
cies below 30 MHz and the FM band approximately en-
compassing 90–110 MHz. The 80 or 100 MHz wide bands
are filtered at the stations into 512 subbands of exactly
156.25/195.3125 kHz using a poly-phase filter. Up to 244
of these subbands can be transported back to the Central
Processor, CEP, giving a maximum instantaneous band-
width of about 39/48 MHz2. There is no restriction on
which 244 of the 512 subbands can be selected to be pro-
cessed at CEP, therefore this bandwidth can be distributed
throughout the entire available 80/100 MHz. Alternatively
it is possible to portion out the bandwidth into multiple
beams. Previously there was a limit of eight per station,
however a recent new implementation of the beam server
software has enabled each of the 244 subbands to be pointed
in a different direction. These subbands can be further di-
vided into narrower frequency channels in CEP as will be
discussed below. The degree of flexibility afforded by these
choices of frequency and beams allows a wide range of high-
time-resolution pulsar-like observations with LOFAR; these
different modes are described in detail in Sect. 5.
1We note that when we refer to dipoles and tiles we are gen-
erally referring to both the X and Y polarisations together, that
is dipole pairs, and we draw a distinction between the two po-
larisations only when necessary.
2This number may further increase if the number of bits used
to describe each sample is reduced.
Table 1. Arrangement of elements in LOFAR stations.
Station Type LBA (no.) HBA tiles (no.) Baseline (km)
Core 2×48 2×24 0.1−1
Remote 2×48 48 1 −10s
International 96 96 ∼100s
Notes. Arrangement of elements in the three types of LOFAR
stations, along with their typical distance from the center of
the array (baseline). In the Core and Remote stations there are
96 LBA dipoles but only 48 can be beamformed at any one
time. For these stations, one can select either the inner circle
or the outer ring of 48 LBA dipoles depending on the science
requirements. The HBA sub-stations can be correlated, or used
in beamforming, independently.
In Table 2 we compare the properties of LOFAR with
those of other telescopes currently operating in (part of)
the same frequency range. LOFAR is the only existing or
planned telescope capable of covering the entire lowest 4 oc-
taves of the radio window (10−240 MHz, above the Earth’s
ionospheric cut-off). In some modes this entire range can be
observed simultaneously (see Sect. 5). LOFAR’s total effec-
tive collecting area and instantaneous sensitivity places it
at the forefront of existing low-frequency radio telescopes,
especially in the range 100 −240 MHz, but collecting area
is only one aspect of LOFAR’s capabilities. As will be de-
scribed in more detail later, LOFAR offers many advantages
over current telescopes through its multi-beaming capabil-
ities, flexible backend, high spatial resolution, large instan-
taneous bandwidth, wide total available frequency range,
ability to track, and ability to observe a large fraction of
the sky (i.e. declinations greater than −30 degrees, see Sect.
5).
3

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
Table 2. Comparison of some telescopes that have operated at frequencies below 200 MHz.
Telescope Area Trec Tsky Frequencies Bandwidth Nbits Beams Transit
(m2) (K) (K) (MHz) (MHz)
Arecibo 30000 5700–5000 47–50 1 Restricted
Cambridge 3.6 ha 36000 1400 81.5 1 1 16 Yes
DKR-1000 5000-8000 18000–500 30–120 1–2 Yes
Gauribidanur 12000 500 30000–12000 25-35 2 2 1 Yes
GMRT 48000 295 276 151 16/32 8/4 2 No
LOFAR (LBA) 75200 500 320000–1000 10–90 48 16 <244 No
LOFAR (HBA) 57000 140–180 630–80 110–240 48 16 <244 No
LWA 1000000 320000–1100 10-88 19.2 8 4 No
MRT 41000 500 276 ∼150 2 2 1 Yes
MWA 8000 150 1400–45 80–300 32 5 16(32) No
Nan¸cay Decametric Array 4000 320000–800 10-100 32 14 1 No
LPA/BSA - Pushchino 20000 110 900 109–113 2.5 12 16(32) Yes
UTR-2 150000 550000–9000 8–40 24 16 5(8) No
VLA 13000 1800 74 1.56 2 1 No
WSRT 7000 400 650–175 110–180 8×2.5 8 1 No
Notes. Telescopes that are observing in (parts of) the same frequency range or have done so in the past are considered. The
bandwidths presented are those that are used for pulsar observations and do not take into consideration what fraction may not
be useful due to radio frequency interference. In the case of LOFAR pulsar observations, only a small fraction of the bandwidth
requires masking (see Sect. 6). Note that the collecting area is the maximum and does not take into account efficiencies or hour
angle dependent effects. The transit instruments such as the Mauritius Radio Telescope (MRT; Golap et al. 1998), Gauribidanur
(Deshpande et al., 1989), and the Cambridge 3.6 ha telescope (Shrauner et al., 1998) have some tracking ability but are usually
limited to observing times of a few minutes. The collecting area of Arecibo is based on an illumination equivalent to a 200 m dish.
The collecting area for the UTR-2 (Abranin et al., 2001) is quoted for a frequency of 20 MHz and the 5-beam and 8-beam modes
are discussed in Ryabov et al. (2010) and Abranin et al. (2001) respectively. The Murchison Widefield Array (MWA) can have
32 single polarisation beams and the area is quoted for a frequency of 200 MHz (Lonsdale et al., 2009). LOFAR can have up to
a total of 244 station beams which equals the number of subbands. The Long Wavelength Array (LWA) (Ellingson et al., 2009)
collecting area is quoted for a frequency of 20 MHz and decreases as λ2. We note that the LWA and MWA collecting areas are
those projected for the final system and are not yet in place. For the LOFAR LBA entry, the effective area is based on using the
outermost antenna configuration at a freqeuncy of 30 MHz, which maximizes collecting area. The HBA collecting area is quoted
for a frequency of 150 MHz. The GMRT (Swarup et al., 1991), VLA (Cohen et al., 2007) and WSRT (Karuppusamy et al., 2011)
are multiple dish interferometers. In some cases, not all parameters are available in the literature and so these entries were left
blank. Tsky is an approximate value calculated on a cold piece of sky and is quoted for the full range of available frequencies at
that telescope.
3. Challenges of observing at low frequencies
As previously mentioned, currently the majority of radio
pulsar observations are done between 300 −2000 MHz be-
cause this frequency range greatly reduces the severity of
several low-frequency observational impediments. Here we
describe these challenges in more detail and explain how
LOFAR is capable of mitigating them.
3.1. Interstellar medium
Simply put, effects in the ISM between the observer and
the pulsar degrade the effective time resolution of the data
and, in severe cases, completely mask short time-scale vari-
ations like the typically milliseconds-wide pulses of pulsars.
Free electrons in the ISM between the pulsar and the Earth
cause the pulsed signal to be both dispersed and scattered.
Dispersion results in the pulses at lower frequencies be-
ing delayed relative to those at higher frequencies. This ef-
fect is directly proportional to the number of free electrons
along the line of sight, which is expressed as the disper-
sion measure (DM); the quadratic frequency dependence
of the delay means the effect is particularly severe when
observing with even 1% fractional bandwidths at frequen-
cies below 200 MHz. For example, at a central frequency of
100 MHz and using a bandwidth of 2 MHz the pulses from
a pulsar with a relatively low DM of 20 pc cm−3will be
smeared out by ∼0.33 s, roughly 10 times the pulse width
of most pulsars. Even with this modest bandwidth and DM
the smearing is sufficient to make the majority of pulsars
undetectable and certainly prevents detailed study of the
emission physics or pulse morphology.
Traditionally this problem has been solved by dividing
the available bandwidth into narrower frequency channels,
which are then delayed relative to each other by an amount
dictated by the DM of the source. Because the maximum
rate at which the data can be sampled is the inverse of the
channel width, there is a limit to the number of channels one
can divide a given bandwidth into, and thus at some point
the resulting sampling time itself becomes too long. As pul-
sars are weak radio sources, large bandwidths are nonethe-
less required to improve the signal-to-noise ratio (S/N) of
the detections. However until the advent of digital systems
it was not possible to form the large number of channels
required to use large bandwidths. A technique to fully cor-
rect for the effects of dispersion in the ISM was developed
by Hankins 1971 but because it is computationally very ex-
pensive, this so-called coherent dedispersion has only seen
regular use in the last 15 years or so. The strong depen-
dence of dispersion on frequency has meant that coherent
dedispersion has only seen limited use at low frequencies
(e.g. Popov et al. 2006b; Karuppusamy et al. 2011). Until
now the effect of dispersion has therefore been a strongly
4

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
limiting factor on the number and types of pulsars which
can be observed at low frequencies.
Furthermore, the ionised ISM is not distributed evenly
and the inhomogeneities along a given line of sight be-
tween the Earth and a pulsar will cause the pulsed signal
to take multiple paths, resulting in the pulse being scat-
tered. Depending on the scattering regime, either strong or
weak (Rickett, 1990), along a particular sight line, the de-
gree of temporal scattering of the pulse profile can scale as
steeply as ν−4.4. Along a given line of sight it also shows
a dependence on the total electron content, i.e. DM, but
there are deviations from this relation of at least an order of
magnitude (Bhat et al., 2004). Considering the same pulsar
and observing frequecy and system as discussed in the first
paragraph of this section, and using the relation between
dispersion measure and scattering from Bhat et al. (2004)
we find that the scattering delay would lie between 0.01 and
1 seconds. The stochastic nature of scattering means that it
is difficult to uniquely correct for it without an underlying
assumption for the intrinsic pulse shape; thus, scattering
becomes the limiting factor on the distance out to which
low-frequency observations can be used for detailed study,
or even detection, of pulsars of given rotational periods.
3.2. Galactic background
Diffuse radio continuum emission in our Galaxy at frequen-
cies below a few GHz is predominantly due to synchrotron
emission from cosmic rays moving in the Galactic mag-
netic field. This emission has a strong frequency dependence
(ν−2.6; Lawson et al. 1987; Reich & Reich 1988) and can
thus be a significant component, or even dominate, the sys-
tem temperature, Tsys, at low frequencies. We note however
that this spectral index does vary over the sky and espe-
cially in the low band it may turn over (Roger et al., 1999).
Moreover, as the effect of the sky temperature on sensitiv-
ity can have a frequency dependence that is steeper than
that of the flux density of some pulsars, pulsars located in
regions of bright synchrotron emission, such as along the
Galactic plane, become more difficult to detect at low ra-
dio frequencies. This is especially relevant as two-thirds of
all known pulsars are found within 5◦of the Galactic plane.
3.3. Ionosphere
LOFAR operates at frequencies just above the Earth’s iono-
spheric cutoff, below which radio waves from space are re-
flected3. In this regime, the ionosphere still plays an impor-
tant role in observations by contributing an additional time
and frequency dependent phase delay to incoming signals.
In particular, separate ionospheric cells can cause differen-
tial delays across the extent of an interferometric array like
LOFAR, greatly complicating the calibration needed to add
the signals from multiple stations together in phase. For a
more detailed discussion of the ionosphere and LOFAR see
Intema (2009) and Wijnholds et al. (2010) and references
therein. The specifics of the challenge for beam formed ob-
servations is described in greater detail in Sect. 6.
3Low-frequency radio frequency interference is also reflected
back to Earth by the ionosphere and could potentially be de-
tected from well below the local horizon.
3.4. Addressing these challenges with LOFAR
The flexibility afforded by the almost fully digital nature
of the signal path and the associated processing power of
LOFAR mean that it can address these observational chal-
lenges better than any previous low frequency radio tele-
scope. As will be discussed in Sect. 6 it is possible to take
the complex channelised data coming from the LOFAR sta-
tions and either further channelise to achieve the required
frequency resolution to correct for dispersion, or, when
higher time resolution is required, it is possible to perform
coherent dedispersion on the complex channels. While it is
not presently possible to correct for the effects of scatter-
ing, we note that variations in the magnitude of scattering
are so large that some pulsars with high DMs will still be
accessible, as evidenced by our detection of PSR B1920+21
with a DM of 217 pc cm−3(Figure 4).
The system temperature of LOFAR is sky dominated
at nearly all frequencies and when combined with the very
large collecting area this makes LOFAR very sensitive de-
spite the contribution from the Galactic synchrotron emis-
sion. The Galactic plane is clearly the hottest region, but
that is only a small fraction of the sky4and as we are most
sensitive to the nearby population it will not greatly affect
the number of new pulsars LOFAR will find (see Sect. 4).
Our detection of PSR B1749−28 (Figure 4), which is only
∼1◦from the direction to the Galactic Centre shows that
observations of bright pulsars are still possible even with
the large background temperature of the Galactic plane.
Moreover, for observations of known pulsars in the direc-
tion of the Galactic plane, the narrow tied-array beams (see
Sect. 5) will reduce the contribution from discrete extended
sources, such as supernova remnants (SNRs), to the system
temperature and thus further improve the sensitivity over
single dish or wide beam telescopes.
In terms of calibrating the station/time/frequency de-
pendent ionospheric phase delays, we will exploit LOFAR’s
multi-beaming capability and its ability to simultaneously
image and record high-time-resolution pulsar data (Sect. 5).
The envisioned scheme is to use a separate station beam to
track a calibration source during an observation, use this to
calculate the required phase adjustments per station, and
to implement these online while observing the main science
target with another beam. This is admittedly a difficult
problem and the solution has yet to be implemented. In
the case of the innermost core stations on the Superterp
however, differential ionospheric delays are unlikely to be a
major problem. This means that using the static phase so-
lutions these stations can be combined coherently without
ionospheric calibration.
4. Pulsar science at low frequencies
Here we give an overview of the pulsar science that can
be done at low radio frequencies with LOFAR. This is not
intended as an exhaustive list of envisioned studies, but
rather as a general scientific motivation for LOFAR’s pulsar
modes.
4Note however that LOFAR’s complex sidelobe pattern
means that the Galactic plane can still contribute somewhat
to the total sky temperature even for observations far from the
plane itself.
5

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
4.1. Pulsar and fast transient searches
There are estimated to be approximately 100,000 actively
radio-emitting neutron stars in the Milky Way (Vranesevic
et al., 2004; Lorimer et al., 2006; Faucher-Gigu`ere & Kaspi,
2006), of which at least 20,000 are visible as radio pulsars
due to fortuitous geometrical alignment of the radio beam
with the direction towards Earth. With a sample of close
to 2,000 known radio pulsars to study, we still have only
a rough idea of this population’s overall properties and of
the detailed physics of pulsars (Lorimer et al., 2006). This
uncertainty is partly a product of the difficulty in disen-
tangling the intrinsic properties of the population from the
observational biases inherent to past surveys, most of which
were conducted at ∼350 MHz or ∼1.4 GHz.5Discovering
a large, nearby sample of pulsars with LOFAR will allow
the determination of the distribution of pulsar luminosi-
ties in the low-luminosity regime, crucial for extrapolating
to the total Galactic population, with the potential for de-
tecting a cut-off in that distribution. Such a survey will also
quantify the beaming fraction, that is what fraction of the
sky is illuminated by the pulsar beams, at low frequencies,
showing how this evolves from the more commonly observed
350/1400-MHz bands. Though a LOFAR pulsar survey will
of course also be observationally biased towards a partic-
ular subset of the total Galactic pulsar population, these
biases are in many ways complementary to those of past
surveys, providing the opportunity to fully characterise the
known population.
Detailed simulations of potential LOFAR surveys were
done by van Leeuwen & Stappers (2010) and show that an
all-Northern-sky survey with LOFAR will find about 1000
new pulsars and will provide a nearly complete census of
all radio-emitting neutron stars within ∼2 kpc. An indica-
tion of the sensitivity of LOFAR for pulsar surveys can be
seen in Figure 2. The limiting distance out to which pulsars
can be detected is governed predominantly by scattering
in the ISM. There are however both pulsar and telescope
characteristics that make low frequency surveys an attrac-
tive prospect. The pulsar beam broadens at low frequencies
(Cordes 1978 find that the width changes with frequency ν
as ν−0.25), which nearly doubles the beaming fraction com-
pared to 1.4 GHz surveys. Furthermore, the large FoV and
high sensitivity of LOFAR mean that such a survey can be
carried out far more quickly and efficiently than any other
pulsar survey, past or present. Even in the Galactic plane,
where the diffuse background temperature will reduce the
sensitivity, the ability to form narrow tied-array beams (see
Sect. 5) will reduce the contribution from sources which are
large enough to be resolved.
The large FoV also affords relatively long dwell times,
improving the sensitivity to rare, but repeating events like
the pulses from RRATs (rotating neutron stars which emit
approximately once in every 1000 rotations; McLaughlin
et al. 2006), while the rapid survey speed will allow multi-
ple passes over the sky. Thus, such a survey is guaranteed to
have a large product of total observing time and sky cover-
age (ΩtotTtot ). This factor is important for finding neutron
stars that, for intrinsic or extrinsic reasons, have variable
detectability, such as the RRATs and intermittent pulsars
5Reconciling the total number of neutron stars and the su-
pernova rate (Keane & Kramer, 2008) also remains an outstand-
ing, related issue, with many fundamental questions still unan-
swered.
(the latter can be inactive for days: Kramer et al. 2006),
those in relativistic binaries (where the acceleration near
periastron can alter the periods too rapidly to allow detec-
tion (e.g., Johnston & Kulkarni 1991; Ransom et al. 2003),
and those millisecond pulsars, MSPs,6which are found in
eclipsing systems (e.g., Fruchter et al. 1988; Stappers et al.
1996). It has recently become apparent that to understand
the pulsar and neutron star populations we need to know
what fraction of pulsars are relatively steady emitters com-
pared with those that pulse erratically like the RRATs.
This complete, volume-limited sample can be extrapo-
lated for modelling the entire neutron star population of
the Galaxy, which then constrains the population of mas-
sive stars and the supernova rate, the velocities and spatial
distribution of neutron stars, and the physics of neutron
stars in general. Chances are that this largely unexplored
population of faint or intermittent radio-emitting neutron
stars will also contain exotic systems – double neutron
stars, double pulsars and possibly even a black-hole pul-
sar binary. Such systems provide the best testing ground
for fundamental physical theories, ranging from solid-state
to gravitational physics (Cordes et al., 2004).
LOFAR is highly sensitive to MSPs and as they are
generally much older than other neutron stars, they have
had more time to leave their birth-place in the Galactic
plane and to become equally distributed in the halo as
well as in the plane. Thus, despite being more easily af-
fected by scattering, the lower number of free electrons
along the lines of sight out of the Galactic plane means
that MSPs are still prime targets for LOFAR. Moreover,
they are bright at LOFAR frequencies with the flux den-
sity spectra of some MSPs remaining steep down to 30
MHz (Kuzmin & Losovsky, 2001; Kramer et al., 1999).
Recent studies have successfully detected about half of
the known Northern MSPs using the relatively insensitive
low-frequency (110 −180 MHz) frontends on the WSRT
(Stappers et al., 2008). Malov & Malofeev (2010) have
also shown a number of detections at frequencies near
100 MHz using the Large Phased Array BSA radio tele-
scope of the Pushchino Radio Astronomy Observatory de-
spite the limited time resolution. It is also noteworthy that
the recently discovered “missing-link” MSP J1023+0038
(Archibald et al., 2009), detected with WSRT at 150 MHz,
could have easily been found by a LOFAR survey, assuming
it was observed out of eclipse. The number of MSPs uncov-
ered at radio wavelengths, often at 350 MHz, by observa-
tions of unidentified Fermi sources (e.g. Ransom et al. 2011;
Hessels et al. 2011) indicates that there are still many more
relatively bright sources to be found. The high sensitiviy of
LOFAR will allow it to detect nearby low-luminosity MSPs
and steep spectrum high-luminosity MSPs which are too
far away to be detected at high frequencies.
LOFAR also has the sensitivity to discover the brightest
sources in local group galaxies (van Leeuwen & Stappers,
2010). If observed face-on and located away from the
Galactic disk, the scatter broadening to pulsars in an ex-
ternal galaxy will be relatively low and thus a LOFAR
survey will have high sensitivity for even rapidly rotat-
ing extragalactic pulsars. For a relatively close galaxy like
6MSPs are believed to be formed in binary systems, where
the neutron star accretes matter from the companion star and
is spun up to millisecond periods, sometimes referred to as re-
cycling.
6

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
M33, LOFAR could detect all pulsars more luminous than
∼50 Jy kpc2. Ten of the currently known pulsars in our
own Galaxy have comparable luminosity (Manchester et al.,
2005). There are at least 20 (dwarf) galaxies for which
LOFAR will have good sensitivity to their pulsar popu-
lation. Complementing searches for periodic signals, de-
tecting the bursts of either giant or RRAT pulses can
equally pin-point pulsars. In particular, the ultra-bright gi-
ant pulses could be visible in even more remote galaxies
(McLaughlin & Cordes, 2003). A survey for extragalactic
pulsars would allow us to investigate if the bright end of
the pulsar distribution in other galaxies differs from that
of our Galaxy, and how that ties into galaxy type and star
formation history. Such pulsars can also feed the under-
standing of the history of massive star formation in these
galaxies and also, if sufficient numbers can be found, they
can be used to probe the intergalactic medium and possibly
constrain or measure the intergalactic magnetic field.
In addition to the discovery of bright pulses from pulsars
in nearby galaxies, an all-sky LOFAR survey will also cast
a wide net for other extragalactic and cosmological bursts
(e.g. Fender et al. 2008; van der Horst et al. 2008; Lorimer
et al. 2007). The large FoV of LOFAR means that only
1200/200 pointings of station beams are required to cover
the full Northern Hemisphere with the HBAs/LBAs at cen-
tral frequencies of 150 and 60 MHz respectively7. Through a
dedicated pulsar/fast transient survey, and regular piggy-
backing on the imaging observations of other projects, it
will be possible to obtain an unprecedented Ωtot ×Ttot fig-
ure of merit. For example, 8000 HBA observations of 1 hr
each (roughly two year’s worth of observations, assuming
50% observing efficiency) would provide 4 hours of all-sky
coverage, meaning that events with rates of only 6/day over
the whole sky could be detected in such a data set. The
parameter space of such rare bursts has never been probed
with such high sensitivity. Moreover there are many options
for performing wide angle rapid shallow searches for fast
transients using either single stations, forming sub-arrays
or forming 244 LBA station beams all at once.
LOFAR will also respond to high-energy X-ray/γ-ray
or high-frequency radio triggers. Most high-energy events
peak later in the radio, providing ample time to follow-up
on such events. Dispersive delay, which can easily be 100s
of seconds in the LOFAR low-band, may also aid in detect-
ing the prompt emission of some bursts. Efforts are being
made to reduce the setup time of a typical observation so
that target of opportunity observations can begin quickly
and automatically. Since LOFAR has no moving parts, re-
pointing is only a matter of reconfiguring the delay correc-
tions used in beamforming. The ultimate goal is to repoint
to any location of the sky and begin new observations in
just a few seconds.
4.2. The physics of pulsar radio emission
The sensitivity and frequency range of LOFAR opens up the
low-frequency window to new studies of pulsar emission. It
is precisely in the LOFAR frequency range where some of
the most interesting changes in pulsar radio emission can
be observed, including significant broadening of the pulse
7Compare for instance with the 1.4 GHz Parkes Multibeam
system, which requires of the order of 35000 pointings to cover
a similar area of sky.
profile, presumably due to a “radius-to-frequency” map-
ping and deviations from this expected relation; changes in
the shape of pulse profile components; and a turn-over in
the flux density spectrum. The frequency range accessible
by LOFAR is also where propagation effects in the pul-
sar magnetosphere are expected to be largest (e.g. Petrova
2006, 2008; Weltevrede et al. 2003). Therefore, simultane-
ous multi-frequency observations (see Figures 15 and 16)
are expected to reveal interesting characteristics of the pul-
sar magnetosphere, such as the densities and birefringence
properties (Shitov et al., 1988), that will ultimately lead
to a better understanding of the emission mechanisms of
pulsars.
4.2.1. Pulsar flux densities and spectra
LOFAR’s large fractional bandwidth is a big advantage for
measuring the low-frequency flux densities and spectra of
pulsars. As discussed in Sect. 5.3 it will be possible to use
multiple stations to observe contiguously from 10 MHz to
240 MHz. With the ability to easily repeat observations,
we can be certain to remove effects such as diffractive scin-
tillation, which may have affected previous flux density es-
timates. It is also the case that the timescale for refrac-
tive scintillation becomes very long at these frequencies and
this will have a modulating effect on the determined fluxes.
However this is typically of smaller amplitude. Repeating
these observations will also allow one to determine whether
the spectral characteristics are fixed in time, or whether
they vary, for any given source, something which has rarely
been done in the past. It is also important to note the role
that variable scattering might play in flux density determi-
nation (e.g. Kuzmin et al. 2008).
So far, there are relatively few pulsars for which spec-
tral information in the LOFAR frequency range is avail-
able (Malofeev et al., 2000). Obtaining a large, reliable
set of low-frequency flux density measurements over the
4 lowest octaves of the radio window is a valuable missing
piece in the puzzle of the pulsar emission process. For in-
stance, the generally steep increase in pulsed flux density
towards lower frequencies is seen to turn over for a num-
ber of pulsars at frequencies between 100 and 250 MHz.
However, the physical origin for this turn over is not yet
clear. Conversely, there are a number of pulsars for which
no such break in the spectrum has yet been seen and stud-
ies at lower frequencies are needed to locate this spec-
tral break (Kuzmin & Losovsky, 2001; Malofeev, 2000).
Furthermore, there is evidence for complexity in the shape
of pulsar spectra (e.g. Maron et al. 2000), which again is
an important signature of the pulsar emission process that
LOFAR’s wide-band data can probe. In addition to their
importance for our understanding of the emission process
itself (e.g. Gurevich et al. 1993; Melrose 2004), pulsar spec-
tra and flux densities are a basic ingredient for estimating
the total radio luminosity and modelling the total Galactic
population of radio pulsars.
In many ways, MSPs show very similar radio emission
properties to those of non-recycled pulsars (e.g. Kramer
et al. 1998); yet, they differ from these by many orders
of magnitude in terms of their rotation period, magnetic
field strength, and the size of their magnetosphere. A cu-
rious and potentially important distinction is that MSPs
tend to show un-broken flux density spectra, continuing
with no detected turn-over down to frequencies of 100 MHz
7

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
10-2
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10-2
10-1
100
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104
105
106
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Flux Sensity (mJy)
Pulsar Period (s)
Fig. 2. Sensitivity curves for 600-s observations using the HBAs (left) and the LBAs (right) with a bandwidth of 48 MHz, compared
to the extrapolated flux densities of 787 known pulsars at 150 and 50 MHz respectively. Flux densities were determined using, if
known, 100 MHz fluxes from Malofeev et al. (2000) (circles) otherwise fluxes at either 400 MHz (triangles) or 1400 MHz (diamonds)
(Manchester et al., 2005), whichever were available, were used and scaled to 150 MHz using a known spectral index or a typical
spectral index of −1.8 (squares). All of these flux densities were scaled by pW/(P−W), where Wis the effective pulse width and
Pis the period of the pulsar, to incorporate broadening of the profile by scattering in the ISM. Wwas determined by combining
the pulse width at high frequencies with the broadening due to scattering in the ISM, based on the model of Bhat et al (2004). If
W > 0.75Pthen the pulsar was deemed undetectable. It was assumed that broadening of the profile due to uncorrected dispersive
smearing was negligible. MSPs are shown by the open symbols and “normal” pulsars by filled symbols. The lines in both cases
correspond to the sensitivity of a single international station (fine dashed line), the incoherent sum of 20 core stations (heavy
dashed line), incoherent sum of all LOFAR stations (filled line) and the coherent sum of 20 core stations (bold line).
(Kramer et al., 1999; Kuzmin & Losovsky, 2001) and lower
(e.g. Navarro et al. 1995). Kuzmin & Losovsky (2001) pre-
sented spectra of some 30 MSPs using measurements close
to 100 MHz. Using the sensitivity, bandwidth and track-
ing abilities of LOFAR, it will be possible to more than
double that number, and to provide measurements with
much wider bandwidths. Such flux density measurements
will also be possible using LOFAR’s imaging ability, even if
severe scattering prevents detailed profile studies. In gen-
eral, however, when scattering is not the dominant effect the
improved time resolution enabled by the coherent dedis-
persion mode of LOFAR should also allow us to resolve
individual profile components in MSP profiles, so that the
average profile spectrum can be compared to that of single
pulse components.
Correlating the spectral properties with other properties
of the pulsar (such as pulse shape, geometrical parameters
or pulse energy distributions) may reveal important physi-
cal relationships. These can then be used to further improve
pulsar emission and geometry models. This is particularly
important as more and more sources are being detected at
high energies with Fermi, placing interesting constraints on
the emission sites (e.g. Abdo et al. 2010b).
4.2.2. Pulse profile morphology
Observations of the average pulse profile morphology of
about 50% of pulsars studied over a wide frequency range
indicate substantial and complex variations which are not
easily understood in the standard model of pulsar emission
(e.g. Lyne & Smith 2004; Lorimer & Kramer 2005). This
simple pulsar model has the plasma and emission properties
dominated by the dipolar magnetic field and is often com-
bined with a model where the emission obeys a “radius-to-
frequency mapping”, such that lower frequencies are emit-
ted further out in the magnetosphere resulting in wider
pulse profiles (Cordes, 1978). However this simple picture is
not the full story as evidenced by the spectral behaviour of
the different pulse components (e.g. Mitra & Rankin 2002;
Rankin 1993). The latter property is in general supported
by observations, and probably corresponds to stratification
of the density in the emission zone although plasma propa-
gation effects may play an important role here as well (see
e.g. Lorimer & Kramer 2005 for a review). The sensitivity
and time resolution possible with LOFAR, combined with
the wide frequency range, will allow us to obtain average
profiles for the majority of the known population of pulsars
in the Northern sky (Figure 2). Additionally, hundreds of
new pulsars, expected to be discovered with LOFAR, can
also be studied in detail; as is demonstrated by the high
quality data from our commissioning observations (Figure
4).
There is also observational evidence that profiles that
are dominated by the outer pulse components at high
frequencies evolve to profiles where the central compo-
nent is comparatively much stronger at low frequencies
(e.g. Lorimer & Kramer 2005). The exact profile evolu-
tion may depend on the distribution of plasma along the
field lines or may be the result of different emission zones.
Indeed, the general picture is much more complex: varia-
tions are seen in the pulse profile with different components
having different spectral indices and new components be-
coming visible (Kramer et al., 1994). By combining LOFAR
data over its wide frequency range with higher frequency
observations we can investigate these properties for a much
larger group of pulsars (see for an example Figure 16).
8

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
10-6
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Fig. 3. Sensitivity curves for the HBAs (left panel) and LBAs (right panel) to single pulses as a function of pulse width at a central
frequency of 150 MHz (HBAs) and 50 MHz (LBAs) and assuming a bandwidth of 48 MHz. Single pulse widths for 787 pulsars were
calculated by assuming them to be one third of the width of the average pulse and then scatter broadened assuming the relationship
of Bhat et al (2004). It was assumed that broadening of the pulse due to uncorrected dispersive smearing was negligible. Single
pulse fluxes were determined based on the assumed pulse width and, if known, 100 MHz fluxes from Malofeev et al. (2000) were
used. Otherwise, fluxes were extrapolated to 150 MHz from either 400 MHz or 1400 MHz measurements (whichever was available,
Manchester et al. 2005) using a known spectral index or a typical spectral index of −1.8. The symbols are the same as in Figure
2. The lines in all cases correspond to the sensitivity of a single international station (fine dashed line), the incoherent sum of 20
core stations (heavy dashed line), incoherent sum of all the LOFAR stations (filled line) and the coherent sum of 20 core stations
(bold line).
A further important probe of the emission mechanism,
the geometry and the plasma properties of the magneto-
sphere is polarisation. So far, there have been only limited
studies of the low-frequency polarisation properties of pul-
sars. In contrast, in simultaneous multiple high-frequency
observations strange polarisation variations have been seen
which point directly to the physics of the emission re-
gions (e.g. Izvekova et al. 1994; Karastergiou et al. 2001).
Polarisation studies with LOFAR have the potential to pro-
vide insight into how the polarisation behaves at these
relatively unexplored low frequencies. LOFAR polarisation
data can determine whether at low frequency the percent-
age of polarisation evolves strongly with frequency and if
there are even more severe deviations of the polarisation po-
sition angle swing from the expectation for a dipole field. It
will also be possible to determine if the polarisation proper-
ties change below the spectral break and whether the sign
changes of circular polarisation seen at higher frequencies
show similar properties at lower frequencies or exhibit no
such sign changes.
4.2.3. Single pulses
The average, or cumulative, pulse profiles of radio pul-
sars are formed by adding at least hundreds of consecu-
tive, individual pulses. While most pulsars have highly sta-
ble cumulative pulse profile morphologies, their individual
pulses tend to be highly variable in shape, and require more
complex interpretations than the average profiles. For ex-
ample, single pulse studies of some pulsars have revealed
quasi-periodic “micropulses” with periods and widths of
the order of microseconds (Cordes et al., 1990; Lorimer
& Kramer, 2005). As such observed phenomena exhibit
properties and timescales closest to those expected from
theoretical studies of pulsar emission physics (e.g. Melrose
2004) studying them can strongly constrain models of the
emission process. Single pulse studies at low frequencies are
particularly interesting, because microstructure (Soglasnov
et al., 1983; Smirnova et al., 1994) and subpulse modula-
tion (Weltevrede et al., 2007; Ul’Yanov et al., 2008) tend to
be stronger there. We also expect density imbalances and
plasma dynamics to be the most noticeable at low radio
frequencies (Petrova, 2006).
In some pulsars, single sub-pulses are observed to drift
in an organised fashion through the pulse window (see
Figure 13), an effect which may either be related to the cre-
ation of plasma columns near the polar cap or to the plasma
properties within the pulsar magnetosphere. Some pulsars
show significant frequency evolution in the properties of
these drifting subpulses and observations at low frequencies,
which probe a very different sight line across the typically
enlarged emission beam, can be used to reconstruct the
distribution of emission within the magnetosphere. Greater
constraints can be obtained by simultaneous observations
at different frequencies, within LOFAR bands but also
in combination with high-frequency facilities. Such exper-
iments will reveal how the radius-to-frequency mapping
manifests itself in the sub-pulse modulation; for example, is
the drifting more or less organised at the low frequencies?
There is some evidence that drifting subpulses are more
pronounced (Weltevrede et al., 2007) and perhaps easier to
study at low frequencies.
In order to gauge the number of pulsars whose single
pulses can be studied with LOFAR, we compare the es-
timated single-pulse flux densities in Figure 3. We show
all known pulsars visible with LOFAR and scale them to
LOFAR sensitivity in both the high band and low band.
While there are a number of assumptions (see caption of
9

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
Fig. 4. A sample of average pulse profiles from LOFAR observations of various pulsars made during the commissioning period.
The dark line corresponds to LOFAR observations made with the HBAs with 48 MHz of bandwidth, divided into 3968 channels,
at a central frequency of 163 MHz. Typically about 12 stations were used and they were combined incoherently. At present there
is no flux calibration for the LOFAR data, hence no flux scale is shown. The majority of the observations are ∼1000 s in duration,
except for PSR B1257+12, which was observed for 3 hr. The dispersion measure smearing is less than two bins for the majority
of the pulsars shown; if the smearing is large, this is indicated by a bar on the left hand side. The lighter line corresponds to
observations made at 1408 MHz by Gould & Lyne (1998) except for the observations of PSR B0943+10 which was at 608 MHz and
PSR B1237+25 which was at 1600 MHz (from van Hoensbroech & Xilouris 1997). All archival profiles are from the EPN database
(www.mpifr-bonn.mpg.de/div/pulsar/data/browser.html); they are shown for illustration purposes and are manually aligned. Both
the LOFAR and EPN profiles have been normalised to the peak intensity. The periods are given in seconds and the dispersion
measures in pc cm−3.
Figure 3) that go into these calculations, we see that on av-
erage we expect to be able to see single pulses from about
one third of visible pulsars in the high band and ten per-
cent of visible pulsars in the low band. This is a significant
increase on what has previously been possible. When com-
bined with LOFAR’s very wide bandwidth and the ability
to track sources, it is clear that LOFAR will allow for a
very rich study of the emission physics of radio pulsars (for
example see Figure 13). Based on the success of observing
radio pulsars using multiple telescopes (e.g. Karastergiou
et al. 2001, 2003; Kramer et al. 2003), we expect that in par-
ticular the combined simultaneous multi-frequency observa-
tions of single pulses at very low and high frequencies will
be extremely useful in confirming or refuting some of the
models and interpretations derived from previous data sets.
The length and quality of the available data will be much
improved, both by being able to follow sources for a much
longer time than previously possible with low-frequency
transit telescopes, and by having much better sensitivity,
frequency coverage, and time resolution. In fact, LOFAR
will also be able to detect single pulses from MSPs, some-
thing which has so far been done for only very few sources
(Edwards & Stappers, 2003; Jenet et al., 2001). This will
allow us to study for the first time if pulsar phenomena
discovered in normal, younger pulsars are also present in
their much older brethren. Such effects include potential
mode changing, nulling and drifting subpulse properties of
MSPs. Understanding the occurrence of such effects will
also be important for high precision timing at higher fre-
quencies, as profile instabilities could, for example, hamper
efforts to detect gravitational waves (Cordes & Shannon,
2010). We note, however, that the high time resolution re-
quirement of these observations compete with the effects of
interstellar scattering which will ultimately limit the num-
ber of sources that can be studied. Nonetheless, we expect
that a few hundred sources can be studied (Figure 3), mak-
ing this a particularly interesting aspect of pulsar studies
with LOFAR.
4.3. Pulsars as probes of the ISM and the Galactic magnetic
field
Pulsars are ideal probes of the ionized component of the
ISM, through the effects of scintillation, scattering, disper-
sion, and Faraday rotation. Most ISM effects become sig-
nificantly stronger towards low radio frequencies, meaning
LOFAR is well placed to study these phenomena in the di-
rection of known pulsars. Furthermore, by greatly increas-
ing the number of known pulsars, a LOFAR pulsar survey
will add a dense grid of new sight lines through the Galaxy.
10

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
As the majority of these pulsars will be nearby or out of
the Galactic plane, the dispersion and scattering measures
of this new sample will improve our model of the distribu-
tion of the ionized ISM and its degree of clumpiness (Cordes
& Lazio, 2002).
Scintillation and scattering are related phenomena
which result in variations of the pulse intensity as a function
of frequency and time, and in broadening of the pulse profile
respectively (e.g. Rickett 1970, 1990; Narayan 1992). They
are caused by fluctuations in the density of free electrons in
the ISM. Scintillation studies have been revolutionised in
the last few years by the discovery of faint halos of scattered
light, extending outward to 10 −50 times the width of the
core of the scattered image (Stinebring et al., 2001). This,
in turn, gives a wide-angle view of the scattering medium
with milliarcsecond resolution, and the illuminated patch
scans rapidly across the scattering material because of the
high pulsar space velocity. Some of the most interesting ef-
fects are visible at low frequencies. The dynamic nature of
these phenomena also fits well with LOFAR’s monitoring
capabilities. In a sense, this scintillation imaging will al-
low the monitoring of the range of interstellar conditions
encountered along a particular sight line. The wide band-
widths and high sensitivities will also allow very precise
measurements of the DM of many pulsars; for MSPs this
could be as precise as 1 : 104or better for an individual
measurement. Such precision, even for normal pulsars, will
enable detailed studies of variations in the DM which can
be used with the scintillation properties to study structures
in the ISM (e.g. You et al. 2007).
Scattering is a spatial effect, whereby the inhomo-
geneities in the ISM cause ray paths to be redirected into
the line of sight with a resulting delay associated with the
extra travel time. Scattering is typically assumed to hap-
pen in a thin screen located midway between the pulsar
and the Earth, so that the turbulence obeys a Kolmogorov
law even though deviations are observed (e.g. L¨ohmer et al.
2004). There is evidence that the scattering does not have
the expected frequency dependence for such a spectrum
and that there is also an inner and outer scale to the tur-
bulence which can be measured (e.g. Armstrong et al. 1981,
1995; Shishov & Smirnova 2002). Low-frequency measure-
ments with LOFAR will allow us to measure (even subtle)
scattering tails for many more pulsars, affording better sta-
tistical studies. The study of individual, moderately scat-
tered but bright pulsars will provide a probe of the distri-
bution of electrons along the line of sight, enabling one to
distinguish between the different frequency dependences of
the dispersive and scattering delays that affect the arrival
time of the pulse (see for example Figure 19). Identifying
such effects has the potential to extract distances and thick-
nesses of scattering screens (Rickett et al., 2009; Smirnova
& Shishov, 2010).
LOFAR can increase the number of known rotation
measures to pulsars, which will place important constraints
on the overall magnetic field structure of the Milky Way,
something that is still not well characterised (e.g. Noutsos
et al. 2008; Han 2009; Wolleben et al. 2010). At LOFAR
frequencies it is possible to also measure the very small ro-
tation measures of the nearby population of pulsars which
provides an unprecedented tool for studying the local mag-
netic field structure. The large number of new sight lines
will also allow statistical studies of small-scale fluctua-
tions of electron density and magnetic field variations down
to the smallest scales that can be probed with pulsars
(.1 kpc). These same techniques will open up new vis-
tas when applied to the first (truly) extragalactic pulsars,
which will be discovered by LOFAR in the survey of local
group galaxies that will be undertaken.
4.4. Pulsar timing
Ultra-high-precision pulsar timing (at the ns−sub-µs level)
is not possible at low radio frequencies because the timing
precision is strongly affected by ISM effects that, currently
at least, cannot be compensated for. However LOFAR’s
large FoV, multi-beaming capability, and the availability
and sensitivity of single stations working in parallel to
the main array enable many pulsars to be timed at suf-
ficient precision and cadence to be of scientific interest.
Regular monitoring of the rotational behaviour of radio pul-
sars plays an important role in understanding the internal
structure and spin evolution of neutron stars and how they
emit. It is also an important probe of the ISM and po-
tentially for the emission of gravitational waves. Measuring
DM variations and modelling the scattering of pulse pro-
files at LOFAR frequencies has the potential to help correct
pulse shape variations at high observing frequencies and
thus reduce systematics in high precision timing.
Rotational irregularities such as glitches are attributed
to the physics of the super-dense superfluid present in the
neutron star core, and so their study allows us to probe
a physical regime far beyond those that can be reached in
laboratories on Earth (e.g. D’Alessandro 1996; van Eysden
& Melatos 2010). These glitches might also trigger heat-
ing of the neutron star surface or magnetic reconnections
which can be studied in either or both gamma- and X-
rays (Tang & Cheng, 2001; Van Riper et al., 1991). They
may also cause sufficient deformation of the neutron star
that it might be a gravitational wave emitter (e.g. Bennett
et al. 2010). To allow these multi-messenger follow-up ob-
servations requires the accurate determination of glitch oc-
curence times which can be achieved through regular mon-
itoring with LOFAR. Moreover, the studies of high-energy
emission from pulsars, which is presently done in detail
by the Fermi and Agile satellites and other telescopes like
MAGIC and H.E.S.S. (e.g. Abdo et al. 2009; Tavani et al.
2009; Aliu et al. 2008; Aharonian et al. 2007), as well as
the search for non-burst-like gravitational waves from pul-
sars (Abbott et al., 2010) can only be performed with the
provision of accurate pulsar rotational histories.
Three relatively recently discovered manifestations of
radio emitting neutron stars, the RRATs, the intermittent
pulsars, and the radio emitting magnetars (McLaughlin
et al., 2006; Kramer et al., 2006; Lyutikov, 2002; Camilo
et al., 2006), can all greatly benefit from the monitoring
capabilities of LOFAR. All of these source types show only
sporadic radio activity, which in some cases is only present
for less than a second per day. As such, a large amount
of on-sky observing is often preferable to raw sensitivity,
making such studies particularly well-suited for individual
LOFAR stations observing independently from the rest of
the array. It is possible that there are neutron stars whose
radio emission is even more sporadic; discovering these will
require potentially several days of cummulative integration
time per sky position, which is achievable as part of the
LOFAR Radio Sky Monitor (Fender et al., 2008).
11

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
So far some 30 RRATs have been discovered, though pe-
riods and period derivatives have only been determined for
slightly more than half of these (McLaughlin et al., 2009).
Detailed studies of these sources are hampered by the fact
that they pulse so infrequently as to make proper tim-
ing follow-up prohibitively expensive for most telescopes.
LOFAR’s multi-beaming capabilities and the separate use
of single stations make such studies feasible. These can de-
termine key parameters like the spin period Pspin and its
derivative ˙
Pspin as well as better characterising the nature
of the pulses themselve. These are all essential for under-
standing the relationship of RRATs to the “normal” pul-
sars.
Unlike the RRATs, the intermittent pulsars appear as
normal pulsars for timescales of days to months, but then
suddenly turn off for similar periods (Kramer et al., 2006).
Remarkably, when they turn off they spin down more slowly
than when they are on, providing a unique and exciting link
with the physics of the magnetosphere. Observing more
on/off transitions will improve our understanding of the
transition process. Furthermore, recent work by Lyne et al.
(2010) has shown similar behaviour in normal pulsars, re-
lating timing noise, pulse shape variations and changes in
spin-down rate. Moreover they demonstrate that with suffi-
ciently regular monitoring, that can be achieved with tele-
scopes like LOFAR, it may be possible to correct the effects
these variations cause on the pulsar timing. This will allow
for the possibility of improving their usefulness as clocks.
Another manifestation of radio pulsars which show
highly variable radio emission are the magnetars, extremely
magnetised neutron stars (Bsurf ∼1014−15 G), which were
thought to emit only in gamma- and X-rays. Recently, how-
ever, two such sources have been shown to emit in the radio,
albeit in a highly variable way (Camilo et al., 2006, 2007).
This emission may have been triggered by an X-ray burst,
and was subsequently observed to fade dramatically. Even
more recently, a radio pulsar was discovered which has a
rotation period and magnetic field strength within the ob-
served magnetar range. This pulsar also shows similarly
variable radio emission properties, as seen for the other
two “radio magnetars”, though no X-ray burst has been
observed (Levin et al., 2010). It is only by monitoring these
sources, and regularly searching the sky, in the radio that
we can understand the link between intermittent radio pul-
sations and high-energy bursts, as well as the lifetime of
these sources as radio emitters.
4.5. New populations
The case for observing pulsars at low frequencies, as dis-
cussed so far, is partly based on our knowledge of the typ-
ical spectral behaviour of the known population. However,
there are pulsars like PSR B0943+10, which have flux den-
sity spectra with spectral indices steeper than α=−3.0
(Deshpande & Radhakrishnan, 1994) and a number of
MSPs which have steep spectra and which do not show
a turn-over even at frequencies as low as 30 MHz (e.g.
Navarro et al. 1995). It is as yet unclear whether these ex-
treme spectral index sources represent a tail of the spectral
index distribution. There is a potential bias against steep
spectrum objects as they need to be very bright at low fre-
quencies in order to be detected in high-frequency surveys.
For example, the minimum flux density of the HTRU, High
Time Resolution Universe, survey with the Parkes telescope
at 1400 MHz (Keith et al., 2010) is 0.2 mJy and for a spec-
tral index −3.0 pulsar the corresponding 140 MHz flux den-
sity would be 200 mJy. In contrast, a source close to the
LOFAR search sensitivity of about 1 mJy would have to
have a flux density no steeper than −0.7 to be detected by
HTRU, suggesting that a large number of pulsars may only
be detectable at low frequencies.
Beyond these regular pulsars with steep spectral in-
dices, more exotic neutron stars are seen to show tran-
sient radio emission. While the anomalous X-ray pulsars
XTE J1810-197 and 1E 1547.0-5408 (Camilo et al., 2006,
2007) have shallow spectral indices and are very dim in
the LOFAR regime, several other AXPs and magnetars
may appear to be only detectable at frequencies near
100 MHz (e.g. Malofeev et al. 2006), offering an intrigu-
ing possibility to study more of these high-magnetic-field
objects in the radio band if confirmed. The potential 100-
MHz detection of Geminga, a prominent rotation-powered
pulsar visible at optical, X- ray and gamma-ray wave-
lengths (Malofeev & Malov, 1997; Kuzmin & Losovskii,
1997; Shitov & Pugachev, 1997), could exemplify a pop-
ulation of neutron stars that may be only detectable at
radio wavelengths with LOFAR. Also, the non-detection
of radio emission from X-ray dim isolated neutron stars
(XDINSs) thus far (Kondratiev et al., 2009), could be due
to the beams of these long-period sources being quite nar-
row at high frequencies and there may be a better chance to
detect them in radio at LOFAR frequencies. In fact, weak
radio emission from two XDINSs, RX J1308.6+2127 and
RX J2143.0+0654, was reported by Malofeev et al. (2005,
2007) at 111 MHz, hence it would be important to con-
firm this detection with LOFAR. This is also the case for
the pulsars discovered in blind searches of Fermi gamma-
ray photons, many of which do not exhibit detectable radio
emission (Abdo et al., 2009, 2010b), as well as the remaining
unidentified gamma-ray sources, which have characteristics
of radio pulsars but which have not yet been detected in
the radio (Abdo et al., 2010a).
5. Observing modes
Here we describe LOFAR’s various “beamformed” modes,
both currently available or envisioned, for observing pulsars
and fast transients. Though we concentrate on the pulsar
and fast transient applications of these modes, we note that
LOFAR’s beamformed modes are also applicable to other
high-time-resolution studies including dynamic spectra of
planets (both solar and extra-solar), flare stars, and the
Sun. We also provide a brief description of LOFAR’s stan-
dard interferometric imaging mode, which can be run in
parallel with some beamformed modes. For a detailed de-
scription of LOFAR imaging, see Heald et al. (2010) and
van Haarlem et al. (2011).
LOFAR is an interferometer with sparsely spaced sta-
tions, distributed in such a way as to produce reliable high-
resolution images. To achieve this, the data from all stations
are correlated8with each other, resulting in a significant in-
crease in the amount of data. To reduce the data rate to an
acceptable level there is an averaging step which reduces the
8Note that the antennas within an individual station are first
summed in phase to form one or multiple station beams on the
sky.
12

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
time resolution of the data to typically about one second, or
longer (though somewhat shorter integrations are possible
in some cases). To sample the combined radio signal at sig-
nificantly higher time resolution than this (tsamp <100 ms),
one has to normally sacrifice spatial resolution, and/or the
large FoV seen by the individual elements, to form a sin-
gle beam pointing in the direction of the source of interest.
It is these so-called beamformed or pulsar-like modes that
we will describe here. The LOFAR Transients Key Science
Project (Fender et al., 2006) will use both the imaging and
beamformed modes to discover and study transient sources.
The imaging mode will probe flux changes on timescales of
seconds to years, while the beamformed modes will probe
timescales from seconds down to microseconds and will re-
visit the same sky locations over the course of days to years.
With the Transient Buffer Boards (TBBs; Sect. 5.5) it will
be possible to form images with high time resolution, but
limited observing durations.
There are many ways in which the various parts of
LOFAR (antennas, tiles, stations) can be combined to form
beams (see Table 3). The almost completely digital na-
ture of the LOFAR signal processing chain means that it
is highly flexible to suit a particular observational goal. In
the following sub-sections we will discuss different options
for combining these signals, to maximise either the FoV, in-
stantaneous sensitivity or to compromise between these two
factors. For the sake of clarity however, we begin by defin-
ing some related terms. An element beam refers to the FoV
seen by a single element, a dipole in the case of the LBAs
and a tile of 4 ×4 dipoles in the case of the HBAs (recall
that these dipoles are combined into a tile beam using an
analog beamformer). The term station beam corresponds
to the beam formed by the sum of all of the elements of
a station. For any given observation there may be more
than one station beam and they can be pointed at any lo-
cation within the wider element beam. A tied-array beam
is formed by coherently combining all the station beams,
one for each station, which are looking in a particular di-
rection. There may be more than one tied-array beam for
each station beam. Station beams can also be combined in-
coherently in order to form incoherent array beams. These
retain the FoV of the individual station beams and have
increased sensitivity compared with a single station.
5.1. Coherent and incoherent station addition
5.1.1. Coherent addition
To achieve the full sensitivity of the LOFAR array it is
necessary to combine the signals from each of the station
beams coherently, meaning that the phase relationship be-
tween the station signals from a particular direction must
be precisely determined. Phase delays between the signals
are generated by a combination of geometric, instrumental,
and environmental effects. The geometric term is simply re-
lated to the relative locations of the stations and the source
and is easily calculated. Instrumental contributions are re-
lated to the observing system itself, such as the length of
cables connecting the various elements. Consequently, the
components in the signal chain are designed to be stable on
timescales that are long compared to the observing dura-
tion. The environmental terms can be much more variable
and are dominated by ionospheric delays at low radio fre-
quencies.
Combining the station signals coherently into tied-array
beams gives the equivalent sensitivity to the sum of the col-
lecting area of all the stations being combined. However, the
resulting FoV is significantly smaller than that of an indi-
vidual station beam because it is determined by the dis-
tance between stations, which for LOFAR is significantly
larger than the size of the stations themselves. The FoVs
for various beam types are given in Table 3 and examples
are shown in Figure 1. The small FoV of tied-array beams
is not a problem for observing known point sources −in
fact, it can be advantageous by reducing the contribution
of background sources −but increases the data rate and
processing requirements when undertaking surveys (see be-
low).
The coherent combination of telescopes for pulsar obser-
vations is regularly used at the Westerbork Synthesis Radio
Telescope (Karuppusamy et al., 2008) and the Giant Meter-
Wave Radio Telescope (Gupta et al., 2000). In both cases
the phase delays between the telescopes are determined by
regular observations of known and unresolved calibration
sources in a process which is often called “phasing-up”. The
timescale between when observations of calibration sources
need to be made depends strongly on the observing fre-
quency, the largest separation between telescopes and the
ionospheric conditions, but range from a few hours to days.
At the low frequencies of LOFAR, phase delays between
stations are very susceptible to the influence of ionospheric
disturbances (e.g. Nijboer & Noordam 2007). This directly
affects how often phasing-up needs to be done, as well as the
maximum separation between stations that can be coher-
ently combined. Fortunately, LOFAR’s flexible signal pro-
cessing provides a number of options to rapidly phase-up (at
least) those stations located in the core (the innermost 2 km
of the array). The first option makes use of LOFAR’s ability
to produce images in near real time. As part of the imaging
calibration, the phase delays between stations are deter-
mined. As it is possible to operate imaging and beamformed
modes simultaneously (see also Sect. 7.1.4) the phases from
the imaging pipeline can be used to coherently combine the
stations, and to update the calibration continuously. If for
some reason this isn’t sufficient, a second option is available
in which it is possible to trade a small amount of observing
bandwidth to form a station beam which points at a strong
calibrator somewhere in the wide FoV of the element beam.
As long as this probes a similar ionospheric patch, this can
be used to monitor the phases.
An important aspect of keeping the signals from the sta-
tions in phase is the time stamp given to each sample. In
the original design, LOFAR used separate rubidium clocks
at each of the stations. These clocks in turn are governed by
their own Global Positioning System receiver, which steers
the rubidium clock drifts to maintain long-term timing sta-
bility. However it was recognised that while the instan-
taneous accuracy was sufficient, there were medium term
drifts which needed to be corrected for. To alleviate the
need for such corrections on the LOFAR Superterp, a new
single clock system has been implemented: the clock signal
from one GPS-governed rubidium clock is distributed to
these 6 stations. The rest of the core stations will be kept
in phase for the coherent addition using either of the two
methods described above.
Once the phase delays have been determined between
the stations, it is possible to use different geometrical de-
lays to form additional tied-array beams. These additional
13

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
Table 3. Comparison of the LOFAR beam-forming modes
Mode Sensitivity FoV Resolution Data Rate FoM
(Norm.) (sq. deg.) (deg) (TB/hr) (Norm.)
High-Band Antennas (HBAs)
Single HBA sub-station 1 / 0.35 18 / 147 4.8 0.3 1
Single Rem. Station 2 / 0.7 10 / 82 3.6 0.3 3
Single Intl. Station 4 / 1.4 6 / 45 2.7 0.3 9
Fly’s Eye 1 / 0.35 1050 / 8400 4.8 20 56
Dutch Inc. Sum 11 / 4 10 / 82 3.6 0.3 77
Intl. Inc. Sum 11 / 4 6 / 45 2.7 0.3 73
Coherent Superterp (94 beams) 12 / 4 18 / 147 0.5 29 1382
Coherent Sum Core (100 beams) 48 / 17 0.4 / 3 0.075 31 3206
Constrained Coherent Core (29 beams) 10 / 3.5 18 / 147 0.9 9 512
Low-Band Antennas (LBAs)
Single Core Station Outer 1 / 0.35 17 / 132 4.6 0.3 1
Single Core Station Inner <1 / <0.35 105 / 840 11.6 0.3 <1
Single Rem. Station 1 / 0.35 17 / 132 4.6 0.3 1
Single Intl. Station 2 / 0.7 26 / 211 5.8 0.3 5
Fly’s Eye 1 / 0.35 660 / 5300 4.6 12 40
Dutch Inc. Sum 6 / 2 17 / 132 4.6 0.3 40
Intl. Inc. Sum 6 / 2 26 / 211 5.8 0.3 44
Coherent Superterp (15 beams) 6 / 2 17 / 132 1.2 4.5 138
Coherent Sum Core (100 beams) 24 / 8.5 3 / 23 0.19 30 2460
Notes. LOFAR beam-formed modes and their (approximate) associated sensitivity, FoV, resolution (i.e. ∆Ω), data-rate, and
survey FoM (see text). High-band (HBA) and low-band (LBA) sensitivities and FoMs have been normalized to that of a single
24-tile HBA sub-station or a 48-dipole Dutch LBA field respectively (Recall that each Dutch LBA field contains 96 dipoles, only
48 of which are used in any particular observation. Unless otherwise stated, we assume the LBA Outer mode is being used. This
mode gives somewhat higher gain, but reduced FoV compared with the LBA Inner mode.). Quantities are quoted assuming one
beam per station (48 MHz bandwidth) and 8 beams per station (6 MHz bandwidth per beam) respectively. FoV (∝λ2
obs) and
resolution (i.e. FWHM of the beam, ∝λobs ) are quoted for a central observing frequency of 150 MHz (HBA, λobs = 2 m) and
60 MHz (LBA, λobs = 5 m). Note that FWHM is taken to be α×λobs /D, where α= 1.3 and Dis the size of a station or
the maximum baseline between combined stations where applicable. As LOFAR stations consist of several square tiles, they are
not perfectly circular; thus, the product of FoV and sensitivity is not constant when station size increases. We have used LBA
(Inner) / LBA (Outer) / HBA station sizes of 32.3 m / 81.3 m / 30.8 m (core), 32.3 m / 81.3 m / 41.1 m (remote), and 65m /
56 m (international, Inner/Outer mode does not apply here). Further empirical beam modeling will likely refine the value of α,
and will somewhat effect the rough values quoted here. Where applicable, we assume that 24 core stations of 2 ×24 HBA tiles
/ 48 LBA dipoles, 16 Dutch remote stations of 48 HBA tiles / 48 active LBA dipoles, and 8 international stations of 96 HBA
tiles / 96 LBA dipoles are available and can be recorded separately if desired. Fly’s Eye mode assumes all Dutch stations - i.e.
48 HBA core sub-stations plus 16 remote HBA stations or 40 LBA fields of 48-dipoles each are used. For the “Coherent” modes,
we assume the maximum number of tied-array beams required to cover the station beam, up to a maximum of 100 (per station
beam), can be synthesized, and that the maximum baseline between stations is 300 m for the Superterp and 2000 m for the entire
Core. The “Coherent Sum Core” mode assumes that all 48 Core sub-stations are combined coherently. The “Dutch Incoherent
Sum” mode assumes that all 40 Dutch stations (24 core / 16 remote) are combined incoherently. The “Intl. Incoherent Sum”
mode assumes that all 8 international stations are combined incoherently. The “Constrained Coherent Core” mode is a hybrid
coherent/incoherent summation in which the two HBA sub-stations of each core station are first summed coherently at station level
before these stations are in turn summed incoherently. The integration time used in each mode is assumed to be the same, though
this would likely differ in practice, especially in the case of wide-field surveys. The data rates assume 16-bit samples (this could
be reduced if desired), summed to form Stokes I, at the maximum possible spectral/time resolution, which for certain applications
can be downgraded by a factor of a few in order to save on disk space and processing load.
tied-array beams can be used to tesselate sections of a sin-
gle station beam, in order to greatly increase the total FoV
for survey-like observations. They can also be formed inside
different station beams to point at many different known
point sources simultaneously, thereby greatly improving ob-
serving efficiency (see Figure 1 for examples of beams). The
processing capability of CEP will allow us to form at least
200 tied-array beams simultaneously, depending on the ob-
serving bandwidth and time resolution used. This allows
LOFAR to achieve rapid survey speed despite the decrease
in FoV caused by the wide distribution of the stations.
Though the resulting data rate is very large, an advantage
of surveying with tens to hundreds of tied-array beams is
that the position of newly found sources is immediately
known to an accuracy of ∼50(when all core stations are
used). This can save significant follow-up observation time
that would otherwise be spent refining the source position.
5.1.2. Incoherent addition
It is also possible to combine the stations without correct-
ing for the phase differences between them. If the signals
are added after detecting them (i.e. computing the power
and thereby losing the phase information) this results in a
so-called incoherent sum. As there is no longer any phase
relation between the signals from the stations, the signals
do not constructively nor destructively interfere. The com-
bined signal is therefore sensitive to signals anywhere within
14

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
the FoV of the station beam, however the sensitivity in-
creases only as the square-root of the number of stations
combined.
As it is not necessary to keep track of the phase relation-
ship between the stations, the incoherent addition can in-
corporate stations which are more widely spread than those
that can be coherently combined. Determining which mode
to use, for any given observing goal, depends on the amount
of collecting area which can be combined coherently, the
number of tied-array beams that can be formed and the
science goals.
As previously discussed in Hessels et al. (2009) the ob-
servational requirements, e.g. sensitivity and FoV, can be
distilled into a simple figure of merit (FoM, see Table 3),
which is generally applicable to a transient survey9(see also
Cordes et al., 2004; Cordes, 2008, and references therein for
a deeper discussion of survey metrics):
F oM ∝A2
eff
Ω
∆Ω
T
∆T.(1)
To be sensitive to transients over a wide range of source
parameter space, including faint and rare events, this FoM
should be maximized. This means maximizing: Aeff , the
effective collecting area being used; Ω, the instantaneous
field of view (FoV); and T, the total time spent observing
the sky. At the same time, adequate spatial (∆Ω) and time
(∆T) resolution are needed to provide reasonable source
localization, for multi-wavelength follow-up and identifica-
tion, and to resolve short timescale phenomena. As it is not
possible to maximize both raw, instantaneous sensitivity
and FoV simultaneously, different modes naturally probe
different areas of transient parameter space (see below).
5.1.3. Coherent versus incoherent addition for different
science cases
Contingent on the particular science goals, pulsar and fast
transient surveys with LOFAR are likely to be performed
using both the coherent and incoherent addition modes,
sometimes in parallel. As discussed by van Leeuwen &
Stappers (2010), a high sensitivity survey of the entire
Northern Sky is preferentially done using the coherent ad-
dition of stations if one can form at least ∼200 tied-array
beams10. Though such a mode provides the best-possible
source localization and sensitivity, this comes at the price
of a much larger data rate (Table 3) and consequently
much greater processing requirements. In comparison, the
wide FoV afforded by the incoherent sum gives it a very
competitive survey speed. However, there are several draw-
backs compared with the coherent mode: source localization
is comparatively poor, as is the rejection of astronomical
backgrounds and radio frequency interference (RFI). Also,
in the incoherent modes much longer observing times are
needed to achieve the same sensitivity and/or FoM (Table
3), which increases the processing load significantly if ac-
celeration searches for binary systems are to be performed
(e.g. Johnston & Kulkarni 1991; Ransom et al. 2003). On
the other hand, longer dwell times may be advantageous for
9We choose to give extra weighting to Aeff. One may also
consider a FoM that scales linearly with Aeff .
10 Current system performance tests indicate that this should
be achievable.
detecting certain types of transients. Nonetheless, the inco-
herent addition mode provides a way to rapidly survey the
whole sky11 with a competitive sensitivity, thereby mak-
ing it very useful for detecting classes of pulsars which can
emit relatively brightly but more erratically, e.g. RRATs,
intermittent pulsars, radio emitting magnetars, and eclips-
ing sources, as well as other transient sources like flare stars,
planets and potentially new classes of transient sources.
Observations of known point sources at high time reso-
lution, where the maximum sensitivity over a given band-
width is required, are best achieved by using the coherent
sum mode. As discussed earlier, even in this mode multiple
sources can be observed simultaneously if multiple station
beams are used at the expense of total bandwidth. Each of
the tied-array beams can be considered as an independent
entity in the LOFAR processing chain and so, e.g., pul-
sar timing campaigns can be run efficiently by observing
multiple sources at once. It is also possible to have beams
pointing at a range of source types, for example a pulsar,
a planet, a flare star, and an X-ray binary all at the same
time, with different data products being produced for each
of the different sources.
5.2. Observing in multiple modes simultaneously
5.2.1. Coherent and incoherent sum modes
LOFAR is flexible enough to produce coherently and inco-
herently added data simultaneously. This provides, at the
incoherent summation sensitivity, the full FoV of a single
station while still having the full coherent sensitivity in the
centre of the FoV. Though even 200 tied-array beams, gen-
erated using all the stations on the core, only cover ∼3%
of the single station FoV, this parallel mode allows one to
both survey the sky at very high sensitivity (over a small
FoV) and over a large FoV (at lower sensitivity) simulta-
neously (Table 3). We note that a greater fraction of the
single station FoV can be covered if we coherently sum the
Superterp stations only, and this may also prove a useful
survey mode. Alternatively, a mix of coherent and inco-
herent addition can be used when adding the stations. For
example, one might choose to coherently sum the stations
in the compact core of the array, e.g. on the Superterp
(Figure 1), and then incoherently add the stations outside
of the core to this. This results in a compromise between
FoV and sensitivity because the core stations have relatively
short baselines between them (Figure 1). It also results in a
more complex combined beam shape. In contrast, groups of
stations can also be used separately as part of sub-arrays,
as discussed below.
5.2.2. Imaging and beamformed modes
As mentioned above, beamformed data can be taken in
parallel with imaging observations. This opens up the op-
portunity for commensal observing where searches for fast
transients can take place in parallel with imaging-based
searches for slow transients or other observations. This will
be done as part of the Transient Key Science Project’s
Radio Sky Monitor, which will image the sky down to 1
11 Using 7 station beams per pointing, only about 200 HBA
pointings are required to cover the entire LOFAR-visible sky
(δ > −35◦).
15

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
second timescales; we will continuously record the incoher-
ent sum of the stations to probe variability on even faster
timescales. In Sect. 7.1.4 (Figure 12) we present the first
such simultaneous imaging/beamformed observation made
with LOFAR taken in 2010 April, when only 7 LOFAR
stations were available for imaging.
5.3. Observing with sub-arrays and single stations
In addition to the incoherent and coherent modes of ob-
serving described above, the array can be split up in vari-
ous flexible ways to accommodate particular science goals.
There are certain pulsar and fast transient science goals
with observational requirements that are best served by
grouping the stations into sub-arrays, or using individual
stations independently. In this way, sensitivity can be ex-
changed for a larger total FoV and/or broader total fre-
quency coverage.
For large sky coverage, LOFAR sub-arrays consisting of
one or several stations (depending on the required sensitiv-
ity) can be used to point at different directions, covering a
large fraction of the sky −potentially the entire LOFAR-
visible sky in the case of the LBAs (Table 3). To maximize
FoV, each sub-array can simply be a single station pointing
in a unique direction. This mode, referred to as Fly’s Eye,
is very similar to that developed for the Allen Telescope
Array (ATA; Welch et al. 2009) and will be used to moni-
tor for rare, very bright, fast transient events. In this mode
the localization of detected bursts can only be achieved
to within the single station FoV, which ranges from a few
square degrees to many hundred square degrees across the
LOFAR band. Much better position determination could be
achieved by triggering as the source is detected and then
using the TBBs to subsequently image the sky to try to
find the source (see Sect. 5.5 for more details).
Another option involves using sub-arrays of stations
tuned to complementary frequencies across the entire
LOFAR band, pointing at the same target. Covering the
full band requires two 48-MHz sub-arrays for the LBAs and
three for the HBAs. Such broad and dense spectral coverage
is crucial for our understanding of the intrinsic processes in
pulsar magnetospheres and propagation effects through the
interstellar medium at low radio frequencies (see Sect. 4).
Individual stations can also be used independently to
conduct targeted observations of known bright objects, or
wide field monitoring of transient events. In this respect, the
international LOFAR stations have the highest sensitivity
and may be available during normal LOFAR observations
when the longest baselines are not in demand. In addition to
the conventional mode where single stations are controlled
through the central LOFAR facilities and send data back to
CEP, the data will be recorded and analysed locally, so that
each station can operate as a stand-alone instrument, or a
group of stations could operate as a coordinated sub-array.
5.4. Polarisation
After careful calibration, LOFAR will enable polarisa-
tion studies of pulsar emission physics and the interstel-
lar medium. A multi-stage process is required to achieve
sufficient polarisation purity. We only summarise the most
relevant issues here and refer the reader to the Magnetism
Key Science project (Beck, 2007) for more details. Both
the LBAs and the HBAs have dual linearly polarised feeds
that are stationary on the ground, meaning that for most
observations the beams will be formed off-axis, potentially
increasing the contribution from polarisation leakage. The
dipoles are aligned South-East to North-West and South-
West to North-East for the x- and y-dipoles respectively
and this means that as a source is tracked across the sky
the projected length of the dipole will change for the major-
ity of sources. Thus, it will be important to accurately cal-
ibrate the gains of the individual dipoles. For all antennas
and stations, Mueller and Jones matrices (Tinbergen, 1996;
Hamaker et al., 1996) need to be determined to correct for
voltage and Stokes-parameter coupling. It is important to
note that the beam patterns are time-variable and different
for each station, however as discussed below it will be pos-
sible to calibrate the system in all but the worst ionospheric
conditions.
To calibrate the individual LBA dipoles and the HBA
tiles in a given station, delay lookup tables that provide
static polarisation calibration will be used initially (see also
Figure 11). However, a multi-level calibration procedure
based on the individual station elements and the stations
themselves is required to ensure polarimetric fidelity as out-
lined in Wijnholds & van der Veen (2009a,b). Combining
the stations to form tied-array beams when including sta-
tions beyond the superterp, will require using dynamic de-
lays provided by the real-time calibration described above.
The LOFAR real-time calibration will correct for the
ionospheric phase delays, but removing the ionospheric
Faraday rotation is more challenging. The limited size of
individual stations means that the ionosphere in most cases
will not cause significant distortions across a station, iono-
spheric Faraday rotation introduces a differential, and fre-
quency dependent, polarisation component in addition to
the differential delays. If the properties of the ionosphere
above each station are determined, the ionospheric Faraday
rotation can be removed by applying a frequency dependent
phase term to the complex polarisations at the level of each
station. The polarisation data of the array can then finally
be used to form 4 independent Stokes parameters, for both
the incoherent sum mode as well as for the phased-array
polarimetric observations, and subsequently processed as
part of the pulsar pipeline using the methods described in
van Straten (2004) and van Straten et al. (2010).
5.5. Transient buffer boards
The TBBs (see Singh et al. 2008) are an exciting and unique
aspect of LOFAR. At each station there is sufficient mem-
ory, RAM, to store the full-bandwidth raw voltage data
from all of the active elements in that station for up to
1.3 s12. These data can then be used to form an image
anywhere in the visible sky, in the case of the LBAs, or
anywhere in the element beam of the HBAs if a trigger,
either external or internal, arrives within 1.3 s of an event
occurring, and causes the data in the buffer to be frozen.
This mode will be used predominantly by the Cosmic Ray
Key Science Project (Falcke & LOFAR Cosmic Ray Key
Science Project, 2007) but will also be an invaluable tool
for the Transients Key Science Project. It will be used to
12 This number may increase in the near future with the addi-
tion of more memory.
16

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
Fig. 5. Schematic diagram of the “online” Tied-Array Beam
(TAB) Pipeline, as it runs on the BG/P. Streaming data from
LOFAR stations are either written directly to disk per station
(“BF RAW Data”) or are read by the BG/P for pipeline pro-
cessing in memory, combining station data into one or multiple
beams, and then writing them to disk (“BF OUT Data”).
localise sources, but to also search for new transient events
at very high time resolution.
The TBBs can also be operated in a mode where data
are saved that has passed through the polyphase filter at
the station. This allows one to choose a reduced bandwidth,
Br, and so significantly increase the amount of time which
can be stored in the buffer: 1.3 s ×100 MHz/Br. This mode
will be very useful when expecting triggers related to short
duration bursts at shorter wavelengths as the unknown dis-
persion delay in the interstellar medium may be many sec-
onds.
6. LOFAR pulsar pipeline
In this section we describe the standard and automated
“pipeline” processing that is applied to the LOFAR beam-
formed data produced by the various modes described in
Sect. 5. We collectively refer to this processing as the
“Pulsar Pipeline” though these data serve a broad range
of scientific interests, not only pulsar science.
The processing is split into two major, consecutive seg-
ments: “online” processing, which is done by a Blue Gene P
(BG/P) supercomputer to streaming data from the LOFAR
stations, and “offline” processing which is further process-
ing of these data on the LOFAR offline cluster (or poten-
tially elsewhere). This hardware is located in Groningen
in the Netherlands and the combination of the BG/P and
offline cluster is known as CEP. The online processing is
chiefly responsible for combining data from multiple sta-
tions into one or multiple beams13, while the offline pro-
cessing is more science-specific and includes, for example,
dedispersion and folding of the data at a known pulsar pe-
riod.
Given that the processing of the raw station data is
done entirely in software, the pipeline is very flexible and
extendable. Here we describe the current implementation
and near future plans with the expectation that this will
continue to evolve to meet new scientific requirements and
to take advantage of increased computing resources in the
future.
6.1. Online processing
Here we briefly describe the online processing chain as it
applies to pulsar-like data. For reference, Figure 5 shows a
simple block diagram of the processing chain. For a more
detailed and general description of the LOFAR correlator
see Romein et al. (2010).
6.1.1. Station data
Each one of the LOFAR stations is capable of sending up
to 244 subbands, that are either 156 or 195 kHz wide, to
CEP in Groningen14. The subbands generated at the sta-
tions are sent as complex numbers representing the am-
plitude and phase, allowing division into smaller frequency
channels if required, and the coherent or incoherent com-
bination of stations. As discussed in Sect. 2 the subbands
can be distributed over a non-contiguous range of frequen-
cies and divided into station beams which can use different,
or the same, set of subbands (see van Haarlem et al. (in
prep.) for further details of station capabilities and data
products). The station signals are fed to the BG/P, which
can combine them into coherent and/or incoherent array
beams, correlate them for imaging or in fact, produce all
three data products at once.
The data flow is arranged so that each input/output
(I/O) node of the BG/P handles all the data from one sta-
tion, which is sent in 1 second sections we refer to as chunks.
These 1 second chunks are then passed, in a round-robin
fashion, to one of the 16 compute nodes attached to each
of the I/O nodes (see Overview of the IBM Blue Gene/P
Project (2008) for a description of the BG/P architecture).
The compute nodes must finish the full online pipeline pro-
cessing, and send the data back to the I/O node to be
written out, before the next 1 second chunk of data arrives.
13 As well as optionally producing correlations for imaging as
described in Sect. 5.
14 We do not consider here the details of the processing that
happens at the stations themselves but refer the interested
reader to de Vos et al. (2009).
17

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
There are sufficient computing resources that while keep-
ing up with the real time processing there is approximately
16 seconds of available computing time for each 1 second
chunk of raw data15.
6.1.2. First transpose
The data arrive at the BG/P such that they are arranged
by station, however in order to efficiently form correlation
products and beams it is necessary to have the data from
all the stations for a particular subband, in the 1 second
chunks, present. The process of reordering these data is
referred to as the “First Transpose” (Figure 5). Before the
transpose, the memory of a particular compute node holds
multiple frequency subbands from a single station; after
the transpose, the same compute node holds subband(s) of
a particular frequency from all the stations being used.
6.1.3. Second polyphase filter
Depending on the particular application, each subband
can be further split into an optional number of channels
(16 −256) via a polyphase filter (PPF). This is referred
to as the “2nd polyphase filter” (2PPF, see Figure 5), be-
cause of the preceding PPF step which occurs at the sta-
tions themselves in order to create subbands (see Sect. 2).
This further channelization serves two purposes: it allows
for the removal of narrow band interference signals in a
way which minimises the loss of bandwidth; it is also re-
quired for incoherently dedispersing the pulsed signal. The
removal of radio frequency interference (RFI) will be dis-
cussed in more detail below, but typically these signals are
significantly narrower than the 156/195-kHz subbands sent
from the stations, so further division into finer frequency
channels is required (see Sect. 8 for details).
As discussed in Sect. 3, any broadband pulsed signal
will be dispersed, and hence smeared in time, by the ISM
between the source and the Earth. This can be corrected by
either incoherent or coherent dedispersion. Coherent dedis-
persion will be used for studying known pulsars, as it pro-
vides the best possible dispersive correction along with the
highest achievable time resolution for a given bandwidth.
Since this technique is computationally very expensive, it is
unfortunately not yet feasible to use coherent dedispersion
for wide-area pulsar surveys, where it would need to be ap-
plied over large numbers of tied-array beams and trial DMs
(thousands of trial DMs in the case of blind searches for mil-
lisecond pulsars). The survey processing chain will therefore
operate on data which has been through the 2PPF and use
computionally efficient incoherent dedispersion algorithms.
Recently we have been able to develop a system which can
perform coherent dedispersion for up to 40 different disper-
sion measures at one time (Mol & Romein, 2011). This will
have application as either a hybrid option where it is com-
bined with incoherent dedispersion for large area surveys,
or using coherent dedispersion for searches of objects where
the DM is constrained to a limited range of values, such as
in globular clusters.
15 This is because there are 244 (i.e. approximately 256) sub-
bands divided over 4096 available compute cores.
6.1.4. Beamforming
It is at this stage that the (optional) beamforming between
multiple stations is done (Figure 5). The stations are com-
bined in one or potentially several of the ways described
in Sect. 5. This is performed on a per subband or even
per channel basis depending on whether the data has been
through the 2PPF. For incoherent beams, the combination
is not done on the complex samples, but instead on the
Stokes parameters (see also below). The geometrical delays
between stations are computed once per second and inter-
polated both in frequency and time, so that each sample is
corrected by a per-sample unique factor. The small number
of possible incoherent beams, only one per station beam,
means that this is not a computationally heavy task com-
pared with the other calculations of the online processing;
nor is the data rate unmanageable (see Table 3). Hence,
incoherent array beams can, and often will, be produced in
parallel to the correlation products needed for imaging or
any other data-products, such as multiple tied-array beams.
In the case of tied-array beams, computational efficiency
is a far greater concern, as up-to several hundred beams
must be synthesized in parallel in order to give this mode
sufficient “survey speed” for all-sky surveys. Beam forming
is performed by applying the appropriate time and phase
correction to the complex samples (Sect. 2). These delays
are applied in three steps: first shifting by integer amounts
of samples, then a phase correction for the center beam
(these are both shared with the imaging pipeline), and fi-
nally a delta-phase correction between the center beam and
each tied-array beam is applied. Determining the phase cor-
rections due to contributions from the ionosphere and the
instrument, which need to be calculated from the data, are
discussed in Sect. 5. An efficient algorithm, partially writ-
ten in assembly code, is used to maximize the number of
beams that can be calculated. The large number of beams
also make the data rate much larger, and the 15 Gb/s de-
signed input limit of the current offline storage cluster plays
the main limiting role on the maximum number of beams
that can be written out. The final LOFAR offline cluster
will have an increase in throughput of about 500%.
To form both incoherent and tied-array beams it is nec-
essary to define a reference phase location. It is strongly
preferable for observations where precise time tagging of
events is required, such as pulsar timing, that this location
be fixed. For LOFAR the phase center of all observations is
now by convention the geographical center of the LBA field
of station CS002, regardless of whether this particular sta-
tion is being used or not. This places the official position of
the LOFAR telescope at (x,y,z) coordinates of (3826577.462
m, 461022.624 m, 5064892.526 m) in the ETRS89 system
(Boucher & Altamimi, 2001). This reference position will
be used for barycentering, pulsar timing, and phasing-up
the array.
6.1.5. Fly’s eye mode
For Fly’s Eye mode, in which the station signals are not
combined into incoherent or tied-array beams at BG/P, the
processing proceeds largely as described above, except that
individual station beams are written out separately using
the same code that stores multiple tied-array beams. In
parallel, it is also possible to generate the incoherent sum
of the station beams if desired. Besides the scientific appli-
18

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
cations made possible by the huge FoV achievable with this
mode, it is particularly handy for debugging issues related
to individual station data because it allows for the compar-
ison of ideally identical stations. Moreover it is useful for
discovering sources of local RFI.
6.1.6. Stokes parameters, downsampling, and re-binning
Prior to beam forming, the individual time samples are
in the form of two 32-bit complex numbers, representing
the amplitude and phase of the Xand Ypolarisations of
the antennas. From these, it is possible to calculate the 4
Stokes parameters, I, Q, U, and V online, assuming inco-
herent dedispersion is to be used, or to record the complex
samples for offline coherent dedispersion. The 4 Stokes val-
ues fully encode the polarisation properties of the signal
and can be used for polarimetric studies. Calibration of the
polarisation products is discussed in Sect. 5.4.
To reduce the data rate one can optionally output only
Stokes I, the total intensity. This is sufficient for many
observations, and is preferable in the case of large-scale
surveys where the data rate is already a major issue.
Depending on the required sampling rate, the data may be
downsampled in time and also scaled to re-pack the 32-bit
Stokes parameters into 16-bit samples (this currently hap-
pens offline but will later be implemented on the stream-
ing data). Application of RFI mitigation strategies to these
streaming data may allow us to further reduce the data to
8-bit samples or less, which will reduce the data rate and
hence potentially allow the formation of more tied-array
beams.
6.1.7. Second transpose
At this stage, the necessary online processing is complete
but the data products are spread across the compute nodes
of BG/P in an order that is contrary to the way they are
typically, and most efficiently, analyzed in further time-
domain based scientific post-processing. Each of the com-
pute nodes contains all of the synthesized beams, but only
for a limited number of subbands. For dedispersion and
other applications the data are best organized such that
all the frequency channels of a particular beam are in one
place. Thus a “Second Transpose” (Figure 5) is carried
out to reorganize the data across the various I/O nodes
of BG/P before it is written to files on the offline cluster.
This Second Transpose is critical for handling large num-
bers of beams in the offline processing. An added advantage
of the Second Transpose is that it will also make it possible
to recombine the subbands using an inverse PPF in order to
obtain a time resolution close to the original ∼10 ns resolu-
tion available before the subbanding of the data at station
level. This mode may be of interest for detailed studies of
giant pulses and for detecting the short radio flashes created
by cosmic rays entering the atmosphere (Falcke & LOFAR
Cosmic Ray Key Science Project, 2007).
6.2. Radio frequency interference excision
The low-frequency window, 10 −240 MHz, observed by
LOFAR contains a range of interfering sources of radio
emission: from commercial radio stations, to weather satel-
lites, to air traffic control communication. To mitigate the
Fig. 6. An overview of the beamformed file structure, encap-
sulated in a hierarchical tree within an HDF5 file. The data
are split into separated sub-array pointing (known in this pa-
per as station beams); within each sub-array pointing are pencil
or tied-array Beams (we will use tied-array beams), along with
coordinates and processing information; each Beam contains a
data structure with either 4 Stokes parameters (or just Stokes I
for total intensity) or, if desired, the amplitude and phase of the
X and Y polarisations separately. Header keywords are stored
within each tree level, depending on their hierarchical relevance
to the rest of the file, as the information is inherited as one
steps downward through the tree structure. See Alexov et al.
(2010a,b) for more details.
influence of this RFI on our data we have implemented
some basic strategies, which we discuss below. We will not
discuss the online RFI mitigation strategies employed by
LOFAR as these will be discussed in detail elsewhere.
Our principal mechanism for excising RFI is to sim-
ply not observe where it is present. The FM-radio band
from about 90–110 MHz is already filtered out within the
LOFAR design (van Haarlem et al. in prep.). The station-
based frequency channelisation of LOFAR to subbands
of either 156 or 195 kHz, combined with the ability to
spread these subbands anywhere within the available 80
or 100 MHz Nyquist zones, already provides the means
to avoid commonly affected frequencies without sacrific-
ing observing bandwidth. As will be shown below (Sect.
7.1.1) many of the interfering sources have bandwidths that
are significantly narrower than the station-based subband
width and so by performing the 2PPF step (Sect. 6.1.3)
to increase the frequency resolution we are further able to
excise RFI with a low impact on the total remaining band-
width. Moreover, the majority of the interfering sources
have a relatively low duty cycle and so only small sections
of time may need to be excised in order to remove the in-
terference.
Our present strategy to identify RFI in our data is such
that for every frequency channel we calculate the quasi-
instantaneous root-mean-square deviation (rms) of data
chunks of duration ∼10 s. The rms calculation is performed
after iterating to remove any bright data points which are
likely to be RFI and biasing the determination of the rms.
If a particular time sample is larger than 6σabove the rms
then it is considered to be RFI. If there are more than 30%
of such samples in the time series, then the whole frequency
19

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
channel is marked as RFI-corrupted and will be excluded
from further analysis. This RFI excision can be run both
in manual mode and as part of our automated pipeline. In
the automated mode we exclude only the corrupted chan-
nels, but leave the strong >6σindividual samples if their
fraction is less than 30%. We do this to avoid removing
any very bright single pulses. This is not problematic since
some short duration sparse RFI in the data will not dramat-
ically impact the data quality. This is because the disper-
sion smearing, even for quite low DM values, is significant
at such low observing frequencies. Thus, during the dedis-
persion process, the RFI, which is at a dispersion measure
of zero as it has not propagated through the ISM, will be
dispersed and thus contribute only extra noise and will not
significantly distort the pulse profile (see Figure 18 for an
example).
6.2.1. Final raw data products
The raw data streaming out of BG/P, in some cases at the
eventual rate of several GB/s, are written to multiple stor-
age nodes of a large offline storage and processing cluster16 .
The data are currently written as almost header-less raw bi-
nary files, but will soon be stored using an implementation
of the Hierarchical Data Format 5 (HDF5)17. The HDF5
file format was chosen because of its flexibility and its abil-
ity to store very large, complex datasets spread over many
separate physical devices.
An overview of the beamformed file structure is shown
in Figure 6; it is too complex to describe here in detail
(Alexov et al., 2010a,b). The hierarchical tree divides the
data into the separate station beams, tied-array beams and
the 4 Stokes parameters (shown) or the X & Y polarisation
amplitude and phase values if complex data is recorded.
For each beam and polarisation parameter, the signal as
a function of time sample and frequency channel is stored
in an array. Metadata fully describing the observation is
stored both at the root level and along the tree where ap-
plicable. For instance, the parameters describing individual
stations or tied-array beams are stored at the same tree
level as these data themselves. Data processing and logging
information are stored in the output beamformed HDF5
files in processing history groups; more general information
is stored at the root of the file, while more specific infor-
mation which pertains only to individual stations or beams
is stored along the tree where applicable. Each beam has
a coordinates group which describes the pointing informa-
tion for that observation; the coordinates are analogous to
the World Coordinate System (WCS) in the FITS format
definition (Alexov et al., 2010a,b).
Though it is expected that LOFAR pulsar data will
be archived in this format, we note that we have already
written a flexible data converter in order to also write
the data into several other community standard formats
like the PRESTO/SIGPROC “filterbank” format and the
PSRFITS format.
16 http://www.lofar.org/wiki/doku.php?id=public:
lofar_cluster#short_lofar_cluster_layout
17 HDF home page: http://www.hdfgroup.org/
6.3. Offline processing
In Figure 7 we show a block diagram of an example pipeline,
in this case the “Known Pulsar Pipeline”, describing the
essential steps in the process. Similar pipelines are being
developed for all the different processing streams as will be
described below.
The raw data from the stations can either be written
directly to the offline storage nodes, which is useful for
sub-arraying or single station experiments, or they can be
passed through the BG/P for further channelization, beam-
forming, and other processing. The starting point for all the
pipelines discussed here is the beamformed data coming out
of BG/P, be that incoherent or coherent beams (though see
Sect. 6.5.1). It is possible to take other data products, such
as imaging visibilities or data dumps from the transient
buffer boards, in parallel, but the processing of those will
be discussed elsewhere.
6.3.1. LOFAR offline cluster
The offline processing pipelines run on the LOFAR Offline
Cluster (LOC), which is the second main part of CEP. The
data writing from the BG/P is distributed across multiple
storage nodes on the LOC, the number of which can be
chosen to match the data rate. The storage and compute
nodes are grouped into sub-clusters, each of which contains
3 storage and 9 compute nodes. The 9 compute nodes of
each sub-cluster can access the data on the 3 storage nodes
via NFS. The current LOC comprises 8 such sub-clusters;
this will soon be expanded as LOFAR approaches full ca-
pability.
After the data are written to the LOC it can either be
passed directly to the long term archive if that is appro-
priate, though that will not normally be done due to the
large volumes of the raw datasets. More usually the data
will pass to one of the pipelines where data processing and
compression happen.
6.3.2. Pipeline framework
All the LOFAR processing pipelines are built to run within
a generic pipeline framework18. This framework takes care
of distributing the processing in parallel over the LOC com-
pute nodes, and provides appropriate logging and error
checking. In the production system, the Pulsar Pipeline
will start processing automatically either during or after
an observation, depending on the data rate and availabil-
ity of processing resources. The allocation of computing
and storage resources, and observing time, will be handled
by software referred to as the LOFAR Scheduler so that
commensal data-taking and processing do not collide. The
Scheduler will also be responsible for passing observation-
specific metadata to the HDF5 header. Further header in-
formation will flow from the observing control systems SAS
(Specification, Administration and Scheduling) and MAC
(Monitoring and Control) into the HDF5 file as well via the
parset (parameter set) as shown in Figure 7.
18 http://usg.lofar.org/documentation/pipeline/
20

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
Fig. 7. Schematic diagram of the overall Known Pulsar Pipeline, as it runs “online” on the BG/P followed by “offline” scientific
processing on the offline cluster. Offline pipeline processing can be run on data directly out of the BG/P or on RFI-filtered data.
Scientific tools such as de-dispersion and folding are performed, along with Stokes calibration to get polarisation and flux densities.
Data products and diagnostic plots are stored in the long term archive.
6.3.3. Software and data access
Wherever possible, all of the different Pulsar Pipelines
are being built around the well-tested, commonly used,
open source software packages available in the pulsar
community (e.g. PRESTO19, PSRCHIVE20, SIGPROC21,
TEMPO22. To allow for efficient development and software
management these have been incorporated into the gen-
eral LOFAR User Software (LUS)23, which uses cmake24, a
cross-platform, open-source build system, to automate the
installation of these packages on a variety of platforms (e.g.
Ubuntu Linux and Mac OS 10.5.X). This cmake installer
may be of interest to others using these same reduction
packages on other systems.
The LOFAR software interface to the HDF5 library is
called the Data Access Library (DAL). It is also available
within the LUS software repository. The DAL allows for
high-level connectivity to HDF5 data, and more specifically
to LOFAR data structures encapsulated within an HDF5
file. The DAL is written in C++; it contains classes which
pertain to all the LOFAR-specific data structures, making
it straight forward to read and write LOFAR HDF5 data.
The DAL is also bound as a Python module, called pydal,
giving users access to the DAL classes via Python. This is
an important set of tools for connecting existing or newly
written processing code to the data.
6.3.4. Radio frequency interference excision
The Known Pulsar Pipeline in Figure 7 shows two paral-
lel processing paths in the offline reduction of LOFAR BF
19 http://www.cv.nrao.edu/∼sransom/presto/
20 http://sourceforge.net/projects/psrchive/
21 http://sigproc.sourceforge.net/
22 http://www.atnf.csiro.au/research/pulsar/tempo/
23 http://usg.lofar.org
24 http://www.cmake.org/
data. The standard path performs dedispersion and folding;
optionally, the pipeline can be run on the same data, but
having performed RFI-excision prior to folding and dedis-
persion. This process filters the data of RFI and then runs
the same tools as run for standard processing. The data
processing time doubles since the pipeline is essentially per-
forming the same processing twice, once on unfiltered data
and a second time on RFI-cleaned files. Once the RFI-
cleaning algorithms have been refined, we expect to only
run the processing once, using the RFI-cleaned data.
6.4. Known pulsar pipeline
The Known Pulsar Pipeline processes data taken in
the following modes: Incoherent Stokes, Coherent Stokes,
Coherent Complex, Fly’s Eye and Commensal Imaging. In
all cases, once the data is on the LOC, the processing done
by the Known Pulsar Pipeline is very similar to standard
pulsar processing (e.g. Lorimer & Kramer 2005). Once the
data have undergone RFI excision it is then dedispersed at
the known DM using either coherent or incoherent dedisper-
sion, depending on the particular source and the scientific
goals. The dispersion-corrected data will then be used to
generate a number of output data products such as dedis-
persed time series, dynamic spectra and folded pulse pro-
files. It is likely that we will simultaneously observe multiple
pulsars, and so multiple instances of the pipeline, with dif-
ferent dispersion and fold parameters, will run in parallel.
The processing capability of LOFAR is such that we are
in general able to produce all of these data products for
each known pulsar observation. These data products are
then flux and polarisation calibrated, stored to the long
term archive as well as being passed to post processing
programs. Multiple pipelines, from multiple separate ob-
servations, can run in parallel depending on the available
computing resources on the LOC.
21

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
0 0.1 0.2 0.3
Time (s)
01234 5
Time (s)
Fig. 8. Observations of giant pulses from PSR B0531+21 in the Crab Nebula observed with both the HBA and LBA. The top
panel shows a “double giant” pulse observed using the incoherent sum of the HBAs from 6 core stations over the frequency range
139 −187 MHz. A section of the timeseries data (black line) is shown indicating a giant pulse which is followed by a second giant
pulse in the next rotation of the pulsar. The fading grey lines show the average pulse profile for this observation repeated four
times. The two giant pulses clearly show the influence of scattering in the ISM and from the Crab Nebula itself. The lower panel
shows a single giant pulse observed with the LBAs from 17 core stations added incoherently over the range 32−80 MHz. Note the
significantly different timescale for the scattering delay compared with the HBA observation.
As well as these scientific data products the pipeline
(Figure 7) will also produce a series of “pulsar diagnostic
plots” e.g. signal statistics, bandpasses, RFI occupancy and
calibration information, which can be used to both calibrate
the data and assess their quality. These will be stored in the
long term archive. In some cases raw data products with
lower time and frequency resolution will also be archived
to allow further processing if needed. This will be particu-
larly valuable when wide FoV modes are being used, as new
sources might later be found in these same observations.
6.5. Periodicity and single pulse search pipeline
As described in Sect. 4, pulsar/fast-transient search ob-
servations are a key aspect of the LOFAR pulsar science
case. These can be divided into at least five types of
searches, each with its own corresponding data-reduction
strategy and science goals. Together, these complementary
approaches span a huge range of source parameter space
(e.g., source brightness, recurrence rate of transients, and
sky location):
–i) Targeted searches of multi-wavelength sources using
one or a few tied-array beams. Here maximum sensitiv-
ity is required over a relatively small FoV.
–ii) All-sky survey using hundreds of tied-array beams.
This is the best option for a high-instantaneous-
sensitivity all-sky pulsar/fast-transient survey.
However, the small FoV of tied-array beams will
limit the possible dwell time to roughly 10 minutes.
–iii) All-sky survey using (multiple) incoherent beams.
This provides a shallower survey in terms of raw sen-
sitivity, but the large FoV of potentially multiple in-
coherent beams makes 1 hour (or longer) dwell times
feasible.
–iv) Wide-area Fly’s Eye searches for rare, bright tran-
sients. Pointing the station beams in different directions
allows even longer dwell times, with reduced sensitivity.
–v) Piggy-back observations using a single incoherent
beam. These intend to maximize the product of total
on-sky observing time and FoV in order to detect the
rarest transient events. Ultimately, this could provide
on the order of a full day’s worth of integration time
over the entire LOFAR-visible sky on the timescale of
22

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
Fig. 9. An initial summary of the RFI situation across our most commonly used sections of the LBA (left-hand panels) and HBA
(right-hand panels) frequency ranges. The upper and lower panels correspond to a frequency channel width of 195 kHz and 12 kHz,
respectively. Each line in the plot corresponds to the percentage of observations in which the particular frequency channel was
affected by RFI.
a few years.
6.5.1. General search requirements and processing
We currently have a basic search pipeline in place that can
deal well with searching a single incoherent beam or a few
tied-array beams. However, modes using hundreds of tied-
array beams require highly optimised processing chains and
the full LOC hardware to be in place, and so we are cur-
rently only in the planning phase of this more complex
mode25. As well as potentially dealing with large num-
bers of beams, the very low frequencies and wide fractional
bandwidths involved in the pulsar searches means that in
the Pulsar Search Pipeline, the number of trial DMs will
be at least 10000, which clearly makes dedispersion one
of the rate limiting steps. We are therefore in the process
of designing an optimal mechanism to efficiently process
pulsar search data within the CEP framework. As well as
considering hybrid coherent/incoherent dedispersion modes
25 Note that this processing challenge is still only a fraction of
what will be required for the SKA.
we are looking at moving some survey processing on to the
BG/P. Distributing and parallelizing the pipeline will be
done by the Pipeline Framework; however, the mechanism
itself of how data processing will be performed and which
algorithms will be used on the LOC are still being deter-
mined.
We now consider the different envisioned search modes
in more detail:
Targeted Searches Targeted searches of known, multi-
wavelength sources require maximum sensitivity over a rel-
atively small area of sky compared with the ∼20 sq. deg
single station beam FoV. This mode will therefore use a
number of tied-array beams, the total required number de-
pending on the size of the source or its positional uncer-
tainty. If the number of beams is significantly less than
needed for the all-sky survey, then the load on BG/P and
the offline cluster will be modest and so simultaneous imag-
ing modes are likely to be run in parallel. For this, and other
modes which use tied-array beams, one beam may be sacri-
ficed to point well away from the target area and be used as
a reference beam for RFI excision. Potential targets for such
23

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
Phase (o)
090
Normalized SNR
01
Time from transit (s)
0 1000 2000
-
1000
-
2000
Fig. 10. A transit observation at a central frequency of 165 MHz
of PSR B0329+54 as it passes through the zenith above core
station CS302. The antennas were combined coherently with-
out further geometrical delay in order to point directly above.
Left: The intensity of PSR B0329+54 as a function of time from
transit and rotational phase. The variations are due to both the
“fringe” pattern created by the coherent addition of the two
HBA sub-stations of station CS302 and the wider single sub-
station beam width. Right: The corresponding normalized S/N
of the pulsar as a function of time from transit.
searches include, e.g., unidentified Fermi γ-ray sources, ap-
parently radio quiet γ-ray/X-ray pulsars, magnetars, and
nearby galaxies or globular clusters.
All-Sky Coherent (Tied-Array) Survey The planned all-sky
tied-array survey will be by far the most data and com-
putation intensive implementation of the pulsar pipelines.
The optimal way to perform the survey is with, at least, a
couple of hundred tied-array beams. A maximal implemen-
tation will result in a data rate of order 23 TB hr−1. We are
currently pursuing two options for the survey processing,
which depend greatly on the mix of computing resources
and storage capacity. If the latter is the most restrictive
then a real-time processing chain may be needed, where
dedispersion, FFT’ing, and candidate selection will be done
in close to real time with only data products related to the
resultant candidates being stored. Alternatively, relatively
short observing sessions will take place with data being
stored and processed at a later date. The latter approach is
preferable, but some hybrid process may be required. Such
a survey provides the highest possible raw sensitivity, but
the dwell time per pointing will have to be modest because
of the limited FoV. Certain classes of intermittent sources
may be better sampled by an all-sky survey using incoher-
ent beams (see below).
All-Sky Incoherent Survey The raw sensitivity of incoherent
array beams (using say all 40 Dutch stations) is roughly 3
times lower than the sensitivity of tied-array beams made
from the combination of all 24 core stations. Nonetheless,
these incoherent beams have over 1000 times larger FoVs,
making shallow all-sky surveys in this mode an attractive
possibility because of the ability to use long dwell times.
Using multiple station beams, the FoV can be increased
even further. For instance, using 7 station beams provides
an instantaneous FoV of ∼170 sq. deg, at a central fre-
quency of about 140 MHz, allowing one to search the en-
tire LOFAR-visible sky (δ > −35◦) with only 200 point-
ings. Even with a 1 hr dwell time, such a survey can be
completed in roughly 1 week of observing time. Shallow
surveys such as the example described above are not only
much more tractable in terms of required observing and
processing time, but they also provide an excellent com-
plement to deep all-sky surveys with higher instantaneous
sensitivity. This is because some intermittent sources, e.g.
the RRATs, may be easier to identify in surveys where the
product of FoV and dwell time is large (assuming the un-
derlying sensitivity is also sufficient).
Fly’s Eye Searches Very bright but rare transients may be
better found by surveys that sacrifice even more raw sensi-
tivity in favor of increased FoV and dwell time. Fly’s Eye
searches will be processed in a similar way to the all-sky sur-
veys, except that different data streams will correspond to
individual station beams pointing in a variety of directions,
instead of different tied-array or incoherent array beams.
There could be up to a couple hundred such beams depend-
ing on how the observing bandwidth is distributed among
individual station beams. This provides, in principle, the
ability to monitor the entire LOFAR-visible sky at once
making LOFAR a truly synoptic radio telescope.
Piggy-Back Searches Piggy-back searches aim to achieve
the maximum possible on-sky time for transients by observ-
ing in parallel whenever possible. This mode will be run as
often as possible when the telescope is doing standard imag-
ing observations or even during tied-array observations. It
will use only one incoherent or a few tied-array beams and
so the computational load is reasonable. The simultaneous
imaging data might be used when interesting candidates
are found. These relatively shallow, but typically wide FoV
observations allow for repeated studies of the same piece of
sky in order to probe variable and transient sources. The
additional processing requirement is small compared with
that required for imaging.
24

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
Fig. 11. Three consecutive 20 minute LBA observations of PSR B1133+16 taken on 2010 Nov 5. In between these observations,
the station calibration was alternately turned off and then back on. The profiles are all scaled to have the same peak height and
as can be seen, the S/N of the cumulative pulse profile decreases by roughly a factor of 3 when the static calibration table is
not applied. Five Dutch stations were combined incoherently in these observations, and as such the S/N difference is the average
calibration gain for all these stations.
Single station processing In Single Station mode, the net-
work streams carrying the data that would otherwise go to
the BG/P need to be redirected to local servers. This in-
volves for the most part the 48 MHz of beamformed data,
which are streamed through 4 UDP26 streams of approxi-
mately 800 Mb s−1each. Members of the LOFAR pulsar
collaboration have developed a hardware and software so-
lution (codenamed ARTEMIS) acting as a single station
backend and capable of recording data for observations of
pulsars and fast transients. This backend has the process-
ing power required to perform a number of relevant opera-
tions in real time, such as RFI excision, channelisation, and
Stokes parameter generation. It is also designed to connect
to fast Graphics Processing Units for real time dispersion
measure searching on the streaming data. Besides pulsars
and fast transients ARTEMIS can be used as a general pur-
pose single station backend. Such an instrument allows ob-
servations to be made either in parallel with ongoing central
LOFAR observations, or allow for local processing when the
station is in local or remote control if there are insufficient
BG/P resources for the required processing. Moreover it al-
lows for the possibility of processing and analysing larger
data sets than can be transferred to the Netherlands.
7. Commissioning results
Since pulsar “first light” in the summer of 2007, when there
were only a few test antennas available, we have been com-
missioning the LOFAR telescope for pulsar observations.
In this time, LOFAR’s sensitivity and functionality have
increased dramatically, as has the data quality (for the pro-
gression, see Stappers et al., 2007; Hessels et al., 2009, 2010;
Stappers et al., 2011). The ability to detect bright pulsars
with just one or a few individual HBA tiles means that pul-
sar observations are useful for measuring the system per-
formance from some of its smallest elements up to the full,
combined array. For instance, pulsar observations can be
used to study the beam shapes of individual or beamformed
elements, verify antenna positions, and measure clock sta-
bility. Here we present highlights from some of these system
tests, followed by associated early science results. These
26 User Datagram Protocol
demonstrate that LOFAR is beginning to function as de-
signed.
Though already impressive, these observational results
provide merely a taste of what LOFAR will ultimately be
capable of doing. The number of completed stations is still
increasing and we have only just begun to take data with
coherently combined stations. Also, proper phase calibra-
tion of the LOFAR stations is currently being implemented
and will increase the raw sensitivity by up to a factor of
2−3 (Figure 11). Therefore, we expect that many of the
observations presented here will be surpassed in the com-
ing year(s) by increasingly sensitive measurements using
the full breadth of LOFAR.
7.1. Observational tests of the system
7.1.1. Radio frequency interference
In order to monitor the LOFAR RFI environment, we have
recorded which frequency channels are badly corrupted in
each given observing session. Collating the statistics from
many observations, we are able to determine which specific
channels are more consistently contaminated by RFI than
others. With this information we can then carefully select
which channels to avoid when choosing the channels to pro-
cess. If whole subbands are seen to be dominated by RFI
we can also not select those when transferring data from
the stations. Figure 9 shows the histograms of corrupted
channels as a percentile of the total number of observations
that included that particular channel. These data do not
show the full LOFAR observing range, as they reflect the
frequency range most commonly observed in these commis-
sioning observations (which was chosen to optimize sensi-
tivity for pulsar observations).
We have used more than 350 observations in the HBAs
and more than 50 observations with the LBAs, spread over
about a 3 month period and occurring during both day
and night-time. We compare the statistics for subbands,
the 195 kHz channels delivered directly by the stations,
with those of the typically 12 kHz channels created by the
second polyphase filter on BG/P. One can see that in the
majority of frequency channels the fraction of observations
affected by RFI is less than a few percent, with a higher
25

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
Fig. 12. Left: LOFAR HBA image (139 −187 MHz) of a 5.
◦5×5.
◦5 field including PSR B0329+54, which is circled. The phase
center of the image was pointed to the brightest nearby source, the 40 Jy: 3C 86. The dynamic range in the image is about 1600
and the resolution about 10000 .Right: Pulse stack of roughly 200 individual pulses from PSR B0329+54, detected in simultaneously
acquired beamformed data. The cumulative profile of these pulses is shown at the top.
total percentage of the band being affected at the low end
of the LBA range. It is also noticeable that in the major-
ity of cases the RFI is unresolved in the broader 195-kHz
subbands, meaning that by further channelising the data
we lose a smaller percentage of the available band27. These
results are extremely encouraging and indicate that in gen-
eral, RFI, if they stay at these levels, will have a limited
detrimental effect on pulsar observations.
7.1.2. HBA sub-station beam shape comparison
The two 24-tile HBA sub-stations associated with each core
station can be used separately or combined coherently at
station level. In order to verify that a truly coherent addi-
tion of the two sub-stations was taking place, and to confirm
that the resulting beam shape agreed with theoretical ex-
pectations, we performed an experiment in which the two
sub-stations were pointed at the Zenith and did not track
a celestial position. Data were then recorded as the very
bright pulsar B0329+54 transited through the beam28. The
resulting “fringe” pattern is shown in Figure 10. As the pul-
sar enters the single-sub-station beam, it becomes visible
and the detected intensity then varies as a function of time
as the pulsar passes through the fan-beam pattern that re-
sults from the coherent addition of the two sub-stations.
A simple comparison of the time between maxima, 500 s,
agrees with what is expected for two sub-stations separated
by 120 m, at a central observing frequency of 165 MHz. This
27 Tests from imaging observations indicate that a good frac-
tion of the narrow-band RFI is only resolved at the 1-kHz level.
28 The latitude of the core of LOFAR is 52.
◦9 and the 5.
◦5 single-
sub-station beam is wide enough to encompass the transit of this
source.
conclusively verifies that the sub-stations are indeed being
added coherently.
7.1.3. LBA station calibration
Beam-server software has been implemented in order to ap-
ply static calibration tables that correct for the remaining
phase differences between the individual elements at each of
the stations (e.g. uncorrected differences in cable lengths).
This is necessary in order to make a true coherent addi-
tion of the station antennas, which maximizes station sen-
sitivity and produces a cleaner station beam shape. These
calibration tables are calculated empirically from 24 hour
calibration runs, whose long length aims to reduce the in-
fluence of ionospheric turbulence and RFI present in indi-
vidual observations. These long term values will only need
to be updated infrequently and will be augmented with a
few minutes of observations on bright calibration sources.
As can be seen in Figure 11, station calibration of the LBA
antennas is now in place and is providing a sizable increase
in raw sensitivity. This is particularly important as LBA
pulsar observations are strongly sensitivity limited. We note
that station calibration for the HBAs has also recently been
implemented.
7.1.4. Simultaneous imaging and pulsar observations
As discussed earlier, the ability to simultaneously image the
sky and record high-time-resolution, beamformed data in-
creases the observing efficiency of the telescope and affords
new scientific opportunities. We have performed a number
of simultaneous imaging and pulsar observations to test the
functionality of this mode. Shown in Figure 12 is a 12 hr
26

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
Fig. 13. A 12 hr observation of PSR B0809+74 made with 4 incoherently added HBA stations in the frequency range 140−164 MHz.
The left-most plots highlight how the S/N varies throughout the observation, mostly due to scintillation but also from the changing
telescope sensitivity with zenith angle. The dashed line marks the time when the pulsar crosses the meridian. The S/N value
shown in the left hand plot is calculated for each 7.2 min integration. The middle plot shows an hour of data when the pulsar most
frequently nulled and exhibited mode changing. The right-most plot zooms in on two long nulls (grey), that are easily distinguished
from the on-pulses (black). Note the quality of the data even in this region when the pulsar was not particularly bright, and at a
high zenith angle, where the sensitivity of LOFAR is reduced by a factor of roughly two.
observation of a field including the bright pulsar B0329+54
(with a flux density at 100 MHz of S100 ∼1 Jy). These
data were taken on 2010 April 19 and included 7 core and
3 remote stations. An initial calibration was made using the
three brightest sources in the field, the brightest of which
was placed at the phase center. These observations have
an odd point spread function (PSF), which is the result of
there being only short and long baselines, with little uv-
coverage on intermediate baselines. Note however that the
image here has been “cleaned”, largely removing this ef-
fect. Current observations are already using significantly
more stations and far better (u, v)-coverage.
7.1.5. Multiple station beams
To demonstrate the ability of LOFAR stations to create
multiple beams on the sky (at the expense of total band-
width per beam), we performed an observation in which
two station beams, of ∼24 MHz bandwidth, each simulta-
neously tracked the pulsars B0329+54 and B0450+55 for
0.5 hr. The system was configured such that the ∼20◦-
wide element beam of the HBA tiles was pointed half-way
between the two pulsars, which are separated by about 12◦
on the sky. The resulting pulse profiles can be seen in Figure
4 and an illustrative graphic can be seen in Hessels et al.
(2010). It is worth noting that this angular separation is far
larger than what is possible to observe with a multi-beam
receiver or a focal plane array. Though the HBA system,
using the full sensitivity, is limited to creating multiple sta-
tion beams within the 20◦-wide tile beams, the LBA dipoles
do not have this extra level of beamforming and each has
a roughly 120◦-wide beam, making it possible to simulta-
neously observe sources almost anywhere above the local
horizon. In fact, such observations have recently been made
in which 6 pulsars distributed across the intantaneous LBA
FoV were simultaneously observed.
7.1.6. Observing with multiple stations.
As demonstrated below, individual LOFAR stations are
sensitive telescopes in their own right. Combining the 24
stations in the LOFAR core will increase sensitivity over
that of a single station by up to a factor of about 5 or 24
depending on whether the addition is incoherent or coher-
ent respectively (see Sect. 2). We have performed a series
of observations comparing the measured S/N of the pul-
sar B1508+55 for different numbers of stations added. The
increase in S/N seen in these profiles agrees well with the
theoretical expectation that the S/N should increase with
the square-root of the number of stations which have been
27

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
incoherently combined (see Hessels et al. 2010). We will
continue to test this relation as more LOFAR stations come
online, especially to verify that coherently added stations
are delivering the expected sensitivity increase (see below).
It is possible that RFI local to individual stations will also
affect the summed signal, although less dramatically if they
are more distant and we are forming the coherent sum, and
so we are exploring options to flag these stations online and
dynamically remove them from the combined beam.
7.1.7. Early (Superterp) tied-array observations
With the installation of a system to provide a single clock
signal to all of the stations located on the Superterp (6 LBA
core fields and 12 HBA sub-stations), the task of calibrat-
ing tied-array beams between these stations was greatly
simplified. By observing a bright calibrator source in imag-
ing mode, it is possible to solve for the phase offsets be-
tween stations and then apply these corrections to future
observations. Experience shows that the phase offsets be-
tween Superterp stations, which should most of the time
see the same ionospheric patch, for the majority of the fre-
quencies we are considering here are sufficiently constant on
timescales of hours to ensure that the gain in the direction
of the pulsar remains optimal. Our commissioning observa-
tions so far have shown that coherent addition produces an
improvement in the S/N compared with the incoherent sum
by a factor of √nstations =√12 = 3.5 as would be expected
for tied-array addition to be working (see Figure 14).
7.2. Early science observations
7.2.1. Pulsars with the LOFAR HBAs and LBAs.
We have detected many of the known, bright (S400 >
50 mJy) northern-hemisphere pulsars using both the HBAs
and LBAs (Stappers et al., 2011). Some pulsars (e.g., PSRs
B0329+54, B0809+74, B1508+55, and B1919+21) are even
bright enough to be visible in 1 hr integrations with individ-
ual HBA tiles. As mentioned above, this proved quite useful
for testing the beam shape, phasing, and the tracking ac-
curacy of single, and later summed, HBA dipole elements.
In the case of the LBAs, roughly a whole Dutch station
(48 active dipoles) is necessary to achieve reasonable S/N
within a 1 hr integration on these same bright pulsars. We
note that single pulses are seen with the LBAs as expected
(Sect. 7.2.7).
As described earlier in Sect. 4.2.3, even individual
LOFAR stations have sufficient sensitivity to be interest-
ing for a variety of scientific applications. For example,
Figure 16 shows the average profiles resulting from the si-
multaneous detection of PSR B1133+16 using 96 active
LBA dipoles with the Effelsberg station (called DE601) and
using the 48 HBA tiles of the Dutch core station (called
CS302).
The recent advent of station calibration for the LBAs
discussed in Sect. 7.1.3 has led to a significant improvement
in sensitivity and this is demonstrated by the observations
of PSRs B0329+54, B0809+74, B0950+08, B1133+16 and
B1919+21 shown in Figure 15. These observations cover
32−−80 MHz, likely making them the widest contiguous
bandwidth observations ever made of radio pulsars at these
frequencies. We have even been able to detect pulsars down
to below 16 MHz (Stappers et al., 2011). This provides us
Fig. 14. The large sensitivity gain given by coherently adding
the station beams is illustrated in these two sample observations.
In each observation, both the incoherent and the coherent sum
of the station beams for all 12 Superterp HBA sub-stations were
recorded simultaneously. The resulting profiles were then scaled
so that the off-pulse noise has a standard deviation of 1. This
comparison shows the expected increase in sensitivity due to
coherent summation and smaller beam size (see text for details).
with an unprecedented view into the evolution of the pulse
profile as a function of frequency, allowing any ambiguities,
for example in aligning pulse components, due to dispersive
effects, to be resolved. The contributions from changing ge-
ometry, spectral index variations, scattering in the inter-
stellar medium, and new components can be separated and
studied with data like these. These data were obtained with
the incoherent sum of 17 LBA stations; once we can form
the coherent addition of all the stations in the LOFAR core
the S/N of these observations will be improved by a further
factor of about six. However we already have data of suffi-
cient quality to start such a study. We note that dispersive
effects are affecting the profiles at the lowest frequencies
shown in Figure 15; however observations with more than
an order of magnitude improvement in effective time resolu-
tion are already possible. Moreover, coherent dedispersion
has now been implemented which further increases the time
resolution we can achieve for the least scattered pulsars.
7.2.2. Giant pulses from the Crab pulsar.
The Crab pulsar, B0531+21, is particularly interesting,
in part because it emits extremely bright “giant pulses”
(Staelin & Reifenstein, 1968; Hankins, 1971). These giant
pulses have the highest brightness temperature of any ob-
served astronomical phenomenon and have been seen at
frequencies from a few 10’s of GHz all the way down to
a few 10’s of MHz (Popov et al., 2006a). LOFAR will be
especially useful for studying the Crab pulsar, and other
young pulsars in supernova remnants, because it will be
possible to form small (arc-second to arc-minute depend-
ing on what fraction of the array is used) tied-array beams
which can potentially partially resolve out these nebulae.
This will greatly increase sensitivity to the pulsations over
the nebular background. There are significant variations in
the scattering timescale with time, as seen by Kuz’min et al.
(2008), and the possibility of frequently monitoring these
over a wide bandwidth with LOFAR allows one to study the
changes in the nebular scattering properties on timescales
ranging from the 33 ms rotation period up to years.
Even before having the ability to form tied-array beams
we have detected giant pulses from the Crab pulsar using
the LOFAR LBAs and HBAs. Figure 8 shows a “double
28

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
giant pulse”, where two consecutive giant pulses are sepa-
rated by only one rotation period. Note that at the observ-
ing frequency of ∼150 MHz the scattering tail of each pulse
is longer than the 33-ms pulse period. We have observed the
Crab on a number of occassions and have already seen sig-
nificant variations in this scattering timescale, indicating
changes in the nebula along our line of sight. As shown in
Figure 8 we have also detected giant pulses with the LBAs
in the frequency range of 32–80 MHz that exhibits a scat-
tering tail that extends to many seconds. Studying the evo-
lution of the scattering of these giant pulses over this sort
of frequency range will be a useful probe of the frequency
scaling laws appropriate for this type of scattering.
7.2.3. Multi-day observations of PSR B0809+74
Previous low-frequency monitoring of pulsars has been
hampered by the limited observing times achievable by
transit instruments or telescopes with equatorial mounts.
In stark contrast to this, we have observed several circum-
polar sources for up to 64 hr continuously, using LOFAR’s
full tracking ability. In Figure 13 we show a 12 hr segment
of one of these observations of PSR B0809+74. This corre-
sponds to 33000 pulses and the full data set to a remarkable
178000 pulses. These observations used the incoherent com-
bination of just 4 core HBA stations and yet show high S/N:
the individual pulses from this pulsar are clearly visible,
and the pulsar’s occasional sudden turn-off, the so-called
nulls, can be clearly distinguished above the low noise floor.
Interspersed by nulls, the individual pulses form a drift
pattern in the time versus rotational phase plane (middle
and right-most panels of Figure 13) −a well known phe-
nomenon that provides important insight into the pulsar
emission mechanism. For understanding the interaction be-
tween nulling and drifting, long integrations have been in-
strumental, together with the occasional, fortuitous boost
in pulsar brightness through scintillation (e.g. van Leeuwen
et al. 2003). Never before has a data set been gathered on
a source like this with such a large number of pulses, over
such a wide bandwidth and at this time resolution. These
data, and more like it, will provide a unique view of the
pulse emission physics continuously over timescale of mil-
liseconds to days and we have already begun such studies.
7.2.4. Observation of the Galactic centre and PSR B1749−28
We have easily detected the bright (S400 = 1.1 Jy) pulsar
B1749−28, which is only 1◦away from the direction of the
Galactic Centre (see Figure 4 for pulse profile). Three as-
pects of this detection are noteworthy. First, LOFAR has
adequate sensitivity to pick out a 1.1 Jy pulsed source
against the high sky background in the direction of the
Galactic Centre (especially considering that the single-
HBA-sub-station beam is very wide at these low elevations).
Second, LOFAR is able to observe bright pulsars at a zenith
angle (ZA) of at least 80◦. At such a high ZA, the sensitiv-
ity of the dipoles is reduced by close to a factor of 6, sim-
ply because of projection. Third, these observations were
made with the incoherent sum of the stations, subsequent
observations will be able to use the coherent sum of the
stations reducing the size of the beam and thus resolving
out part of the bright Galactic plane and further improving
the sensitivity. Furthermore, RFI may be more pernicious
Fig. 16. PSR B1133+16’s average pulse profile observed si-
multaneously over nearly 8 octaves in frequency. LOFAR ob-
servations were acquired in both the LBA (42 MHz, station
DE601) and HBA (140 MHz, station CS302) bands and were
supplemented by contemporaneous observations with the Lovell
(1524 MHz) and Effelsberg (8300 MHz) telescopes.
for observations at higher ZA. Nonetheless, this detection
clearly demonstrates that it will be possible to monitor the
Galactic Center for bright fast transients (though scatter-
ing will remain a major limitation).
7.2.5. Millisecond pulsars
Though scattering strongly limits the effective time resolu-
tion achievable at low observing frequencies, LOFAR is still
highly capable of observing nearby MSPs, which are only
mildly scattered29. Figure 4 shows the detections of the 16-
ms pulsar J2145−0750 and the 6.2-ms pulsar B1257+12.
These pulsars both have low DMs (∼10 pc cm−3), which
made these detections possible without the need for coher-
ent dedispersion. Despite the low DM however, one can still
see that the profiles are broadened by dispersive smearing
within the channels. As discussed earlier, creating narrower
channels is possible but will come at the expense of the time
resolution of the samples, such that the narrow pulse profile
will still be poorly sampled. To circumvent this problem, we
have implemented coherent dedispersion on the BG/P itself
to remove intra-channel dispersive smearing and to open
the possibility of observing MSPs with much larger DMs.
An example of the results of observations in this mode is
shown in Figure 17. We will soon be able to do this over
larger bandwidths and LOFAR will then be able to provide
measurements that eclipse those achieved in Stappers et al.
(2008).
7.2.6. Simultaneous LBA/HBA observations
Sub-arraying and independent simultaneous observations
make it possible to use multiple LOFAR stations to cover a
larger frequency band than the maximum 48 MHz instan-
29 MSPs that are further away may also have low scattering
measures in some cases, as the general correlation between dis-
tance, dispersion measure, and scattering shows large deviations
from the general trend, see Bhat et al. (2004).
29

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
Rotational Phase Rotational Phase Rotational Phase Rotational Phase Rotational Phase
Fig. 15. A sample of average profiles of five pulsars observed with the LBAs using a total of 48 MHz with a channel bandwidth
of 12.2 kHz and sampled at a rate of 1.3 ms. In all cases 17 stations were combined incoherently and the observation duration
was 2 hr, except for PSR B1919+21 which was observed for 1 hr. The lower 8 profiles correspond to observations with 6 MHz of
bandwidth centered at frequencies, from lowest to highest, of 35, 41, 47, 53, 59, 65, 71 and 77 MHz respectively. The top profile
shows the summed profile from all 8 bands. At present there is no flux calibration for the LOFAR data hence no flux density
scale is shown and profiles have been normalised to the peak intensity. The bar on the left hand side indicates the smearing due
to dispersion across the 12.2 kHz channels and is shown only when it exceeds 3 ms. The periods are given in seconds and the
dispersion measure in pc cm−3.
taneously available from any one station. In this way, it is
possible to almost30 completely cover the 10−240 MHz ra-
dio band (see Sect. 2). In 2009 December we performed the
first LOFAR observations in which the LBA and HBA sys-
tems were used simultaneously. This was achieved by set-
ting the Dutch core station CS302 in the HBA mode and
the German station at Effelsberg (DE601) in the LBA mode
and running two independent observations in parallel on the
BG/P. In addition, the 76 m Lovell and 100 m Effelsberg
single dish telescopes were used concurrently to record at
1.4 GHz and 8.3 GHz respectively. Figure 16 shows the cu-
mulative pulse profile of PSR B1133+16 observed in four
frequency bands centered at 42, 140, 1400, and 8300 MHz.
In these initial test observations, only one station could be
used for each of the LBA/HBA bands; in the future it will
be possible to combine multiple stations to boost sensitiv-
ity. These data show how we can further extend the studies
of the ISM and pulsar emission physics discussed in Sect.
7.2.1.
7.2.7. LBA detections of “Anomalously Intensive Pulses”
Another example of a phenomenon which may only be
visible at low frequencies are the “anomalously intensive
pulses” (AIPs) reported by Ulyanov et al (2006) from five
pulsars with low DMs, at decameter wavelengths with the
UTR-2 radio telescope. Currently, 6 pulsars are known
to emit such strong sporadic pulses at frequencies below
30 One can in principle observe in the FM band, but the data
are very strongly affected by the interference from radio stations.
35 MHz (Ulyanov et al., 2006, 2007). These pulses are
10−15 ms wide with energy exceeding 10−100 times the
energy of the average profile. The emission is seen to be
quite narrow band with the emission typically ∼1 MHz
wide or seen in a few narrow 1.5–5 MHz frequency chan-
nels. These pulses are very intriguing but are still not care-
fully studied over a broad bandwidth. Such a study can
shed light on a possible link between AIPs and similar phe-
nomena observed at higher frequencies, such as giant pulses
(e.g., Popov et al. 2006a; Soglasnov et al. 2004) giant mi-
cropulses (e.g. Cairns et al. 2004; Smirnova 2006), and spiky
emission (Weltevrede et al., 2006). LOFAR can excel in this
area, and Figure 18 shows several examples of bright single
pulses detected from PSR B0950+08.
7.2.8. Interstellar medium
As well as dispersion, several propagation effects have been
proposed which could cause a frequency dependent delay
in the arrival time of pulses. Many of these effects have
not, as yet, been directly detected or studied. They include
refractive delays, DM variations, delays associated with
pulse broadening from scattering (Foster & Cordes, 1990),
propagation effects from within the pulsar magnetosphere
(Michel 1992), and super dispersion (Shitov and Malofeev
1985; Kuz’min 1986; Shitov et al 1988; Kuz’min et al 2008).
Many of these effects are strongly frequency dependent,
with scaling indices between ν−3and ν−5. LOFAR is ide-
ally suited for studying these types of effects and Figure 19
shows a frequency-phase greyscale of simultaneous observa-
tions with the HBAs and LBAs which were dedispersed to
30

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
0
2e+07
4e+07
6e+07
8e+07
1e+08
0 0.5 1 1.5 2 2.5 3
Relative Amplitude
Time (s)
0
2e+07
4e+07
6e+07
8e+07
1e+08
0 0.5 1 1.5 2 2.5 3
Relative Amplitude
Time (s)
0
1e+07
2e+07
3e+07
4e+07
0 0.5 1 1.5 2 2.5 3
Relative Amplitude
Time (s)
0
1e+07
2e+07
3e+07
4e+07
0 0.5 1 1.5 2 2.5 3
Relative Amplitude
Time (s)
Fig. 18. Observations of PSR B0950+08 made using the LBAs (left) and HBAs (right). The top panels show sections of the
timeseries after dedispersion to the pulsar DM of 2.96 pc cm−3. We easily detect single pulses and one can see the wide range in
intensity of the pulses, in particular in the low band. In the lower panels the same timeseries are shown but without dedipsersion,
the so-called DM zero timeseries. In both the LBAs and HBAs we can see evidence for narrow spikes of RFI at zero DM, however
these are clearly broadband in nature and have been completely dispersed beyond a detectable level at the dispersion measure of
the pulsar. We note that the broad features seen in the DM zero timeseries of the HBAs (e.g. near 2.5 s in the lower-right panel)
are due to the pulsar itself. The ordinate axes corresponds to an arbitrary amplitude but is preserved in the top and bottom plots.
a DM of 26.768 pc cm−3. This observation corresponds to
observing over more than two octaves simultaneously and
so any deviation from the ν−2law which is greater than
∼100 ms should be visible over this large bandwidth and
at these low frequencies. Once the effects have been identi-
fied we should be able to constrain how much of an impact
they have on pulsar timing at higher frequencies and also
extract information about the composition of the ISM.
8. Conclusions and future prospects
We have shown that LOFAR will provide a massive im-
provement in our ability to study pulsars and fast tran-
sients in the lowest frequency range observable from Earth
(10 −240 MHz). Many of these modes will also find ap-
plication for observations of solar and extra-solar plan-
ets, flare stars and other time variable sources. In par-
ticular, we have discussed how wide-band, low-frequency
pulsar observations will address the nature of the still enig-
matic pulsar emission mechanism (Figure 16) and will pro-
vide a useful probe of the ISM (Figure 19). LOFAR also
promises to discover many new pulsars and fast transients
through the combined power of its large collecting area
and (multiple) large FoVs. Through a dedicated all-sky
pulsar/fast-transient survey, a dedicated “radio-sky mon-
itor”, and regular piggy-backing on imaging observations,
LOFAR can revolutionize our understanding of the pop-
ulation of rare/weak radio transients by charting parts of
parameter space effectively inaccesible to traditional radio
telescopes.
Though the LOFAR antennas are still in the process
of being deployed in the field, the commissioning of pul-
sar observing modes has been underway for over 3 years,
with a steady increase in the level of activity and re-
sults. Many of the desired observing modes are now func-
tional, and have resulted in the observations presented
here. While LOFAR is still in its operational infancy, it
has already demonstrated several of the capabilities and
technologies that make it an important precursor to the
Square Kilometre Array (SKA). This includes the use of
multiple digital beams to track sources widely separated
on the sky (Figure 4), simultaneous high-spatial and high-
time-resolution observations (Figure 12), and large instan-
taneous fractional observing bandwidth (Figure 16).
With these successes in hand, we are focussed on contin-
uing development of the pulsar pipelines in order to take full
31

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
Fig. 17. For comparison, two consecutive 20-minute observa-
tions of the 1.88-ms pulsar J0034-0534 at a central observing
frequency of 143 MHz and with a spectral resolution of 12kHz
(corresponding time resolution of 82µs) are shown. The data for
the “Incoherent” profile has only been corrected for dispersion
by applying a frequency dependent time delay to the spectral
channels in the offline post-processing. In contrast, the data for
the “Coherent” profile has also been coherently dedispersed on-
line in order to remove the intra-channel dispersive smearing.
The increased effective time resolution of the coherently dedis-
persed data is evident from comparing the two profile morpholo-
gies; e.g., two profile components are visible in the “Coherent”
profile which are washed out in the “Incoherent” profile. The
off-pulse noise has been scaled to the same level to facilitate
comparison of the S/N. The effective time resolution is repre-
sented by the two horizontal bars on the left-hand side, and re-
flects the intra-channel smearing in the case of the incoherently
dedispersed profile.
advantage of LOFAR’s flexible data acquisition capabilities.
Along with these opportunities come several challenges.
The combination of many thousands of antennas, each
with “all-sky” FoV, is key to LOFAR’s design. Applying
proper phase corrections to add these elements and stations
in phase remains one of our biggest challenges. In partic-
ular, the lack of ionospheric calibration has limited us to
using stations within the Superterp, or adding stations in-
coherently. We are tackling this problem as part of a larger
effort to provide real-time inonospheric calibration for the
purposes of generating short-timescale images that can be
fed into a transient detection pipeline.
Even with coherently summed, “tied-array” beams in
hand, the beam pattern itself will remain very complex and
requires careful study. As we have already seen from blind
searches of our data, the beam response of the combined ar-
ray will often allow bright pulsars to creep into observations
nominally pointed far away from their position. While this
might have some interesting applications, this effect is a
particularly pernicious form of interference for a blind pul-
sar survey and requires careful treatment so that weak pul-
sars can still be found in the same data. This issue becomes
even more delicate in the case of one-off transient events.
The imaging capability of LOFAR will certainly help allevi-
-0.1 0 0.1 0.2 0.3
60
80
100
120
140
160
180
Frequency (MHz)
Delay (s)
Fig. 19. An observation of PSR B0329+54 from December 2009.
Data were taken using the HBAs and LBAs simultaneously.
The low band data was taken using the Effelsberg international
station (DE601) with 36.328125 MHz of bandwidth between
42.1875 MHz and 78.3203125 MHz. The high band data was
taken using a single core station (CS302) with a bandwidth of
48.4375 MHz between 139.0625 MHz and 187.5 MHz. The figure
shows no significant deviation from the ν−2cold dispersion law.
ate this problem, but care is nonetheless needed in ascribing
positions to such events.
To maximize data quality and time resolution, stud-
ies of known pulsars should use coherent dedispersion. For
LOFAR, this is true even for slowly rotating pulsars because
the number of channels required for incoherent dedispersion
quickly becomes so large as to degrade the time resolution
of the data to an unacceptable level. It is already possible
to apply coherent dedispersion offline to LOFAR data that
is recorded as complex samples, but this mode requires a
very large data rate and large amounts of offline processing.
We have now also developed online coherent dedispersion
(on BG/P) in order to circumvent these issues for a range
of pulsars and DMs.
32

B. W. Stappers et al.: Observing pulsars and fast transients with LOFAR
An all-sky pilot imaging survey with LOFAR is planned
in order to address many of the outstanding calibration is-
sues, such as beam pattern and flux density scale. This sur-
vey has as primary purpose the creation of a low-frequency
sky model for future LOFAR imaging and calibration. In
addition to the obvious benefits that better calibration will
have for pulsar observations, this survey will also provide a
platform for a commensal all-sky shallow survey using an
incoherent sum of the station beams. This will be an op-
portunity for us to further automate and refine our data
acquisition and reduction pipelines and to get the first real
glimpse at what an all-sky LOFAR survey for pulsars and
fast transients will offer.
9. Acknowledgements
LOFAR, the Low Frequency Array designed and con-
structed by ASTRON, has facilities in several countries,
that are owned by various parties (each with their own
funding sources), and that are collectively operated by
the International LOFAR Telescope (ILT) foundation un-
der a joint scientific policy. The data in Figure 16 based
on observations with the 100-m telescope of the MPIfR
(Max-Planck-Institut f¨ur Radioastronomie) at Effelsberg
and the Lovell Telescope at Jodrell Bank Observatory.
Ben Stappers, Patrick Weltevrede and the Lovell observa-
tions are supported through the STFC RG R108020. Jason
Hessels is a Veni Fellow of The Netherlands Organisation
for Scientific Research (NWO). Joeri van Leeuwen and
Thijs Coenen are supported by the Netherlands Research
School for Astronomy (Grant NOVA3-NW3-2.3.1) and by
the European Commission (Grant FP7-PEOPLE-2007-4-
3-IRG #224838). Aris Karastergiou is grateful to the
Leverhulme Trust for financial support. Ralph Wijers ac-
knowledges support from the European Research Council
via Advanced Investigator Grant no. 247295. LVEK is sup-
ported by an ERC ST Grant.
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