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arXiv:0810.1676v1 [astro-ph] 9 Oct 2008
1
Astronomy & Astrophysics
manuscript no. 1510˙v10 c
ESO 2008
October 9, 2008
Letter to the Editor
AGILE detection of intense gamma-ray emission from the blazar
PKS 1510-089
G. Pucella1, V. Vittorini1,2, F. D’Ammando1,3, M. Tavani1,3, C. M. Raiteri4, M. Villata4, A. Argan1, G. Barbiellini5,
F. Boffelli6,7, A. Bulgarelli8, P. Caraveo9, P. W. Cattaneo6, A. W. Chen2,9, V. Cocco1, E. Costa1, E. Del Monte1, G. De
Paris1, G. Di Cocco8, I. Donnarumma1, Y. Evangelista1, M. Feroci1, M. Fiorini8, T. Froysland2,3, F. Fuschino8,
M. Galli10, F. Gianotti8, A. Giuliani9, C. Labanti8, I. Lapshov1, F. Lazzarotto1, P. Lipari11, F. Longo5, M. Marisaldi8,
S. Mereghetti9, A. Morselli12, L. Pacciani1, A. Pellizzoni9, F. Perotti9, P. Picozza12, M. Prest13, M. Rapisarda14,
A. Rappoldi6, P. Soffitta1, M. Trifoglio8, A. Trois1, E. Vallazza5, S. Vercellone9, A. Zambra1, D. Zanello11,
L. A. Antonelli15, S. Colafrancesco15, S. Cutini15, D. Gasparrini15, P. Giommi15, C. Pittori15, F. Verrecchia15,
L. Salotti16, M. F. Aller17, H. D. Aller17, D. Carosati18, V. M. Larionov19, R. Ligustri20
1INAF/IASF–Roma, Via del Fosso del Cavaliere 100, I-00133 Roma, Italy
2CIFS–Torino, Viale Settimio Severo 3, I-10133 Torino, Italy
3Dip. di Fisica, Univ. di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, I-00133 Roma, Italy
4INAF, OATo, Via Osservatorio 20, I-10025 Pino Torinese (Torino), Italy
5Dip. di Fisica and INFN Trieste, Via Valerio 2, I-34127 Trieste, Italy
6INFN–Pavia, Via Bassi 6, I-27100 Pavia, Italy
7Dip. di Fisica Nucleare e Teorica, Univ. di Pavia, Via Bassi 6, I-27100 Pavia, Italy
8INAF/IASF–Bologna, Via Gobetti 101, I-40129 Bologna, Italy
9INAF/IASF–Milano, Via E. Bassini 15, I-20133 Milano, Italy
10 ENEA–Bologna, Via dei Martiri di Monte Sole 4, I-40129 Bologna, Italy
11 INFN–Roma “La Sapienza”, Piazzale A. Moro 2, I-00185 Roma, Italy
12 INFN–Roma “Tor Vergata”, Via della Ricerca Scientifica 1, I-00133 Roma, Italy
13 Dip. di Fisica, Univ. dell’Insubria, Via Valleggio 11, I-22100 Como, Italy
14 ENEA–Roma, Via E. Fermi 45, I-00044 Frascati (Roma), Italy
15 ASI–ASDC, Via G. Galilei, I-00044 Frascati (Roma), Italy
16 ASI, Viale Liegi 26, I-00198 Roma, Italy
17 Department of Astronomy, University of Michigan, U. S.
18 Armenzano, Astronomical Observatory, I-06083 Assisi (Perugia), Italy
19 Astron. Inst., St-Petersburg State University, Russia
20 Circolo AStrofili Talmassons, Via Cadorna 57, I-33030 Talmassons (Udine), Italy
received; accepted
ABSTRACT
Context.
We report the detection by the AGILE (Astro-rivelatore Gamma a Immagini LEggero) satellite of an intense gamma-ray
flare from the source AGL J1511-0909, associated with the powerful quasar PKS 1510-089, during ten days of observations from 23
August to 1 September 2007.
Aims.
During the observation period, the source was in optical decrease following a flaring event monitored by the GLAST-AGILE
Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT). The simultaneous gamma-ray, optical, and radio coverage
allows us to study the spectral energy distribution and the theoretical models based on the synchrotron and inverse Compton (IC)
emission mechanisms.
Methods.
AGILE observed the source with its two co-aligned imagers, the Gamma-Ray Imaging Detectorand the hard X-ray imager
Super-AGILE sensitive in the 30 MeV ÷50 GeV and 18 ÷60 keV bands, respectively.
Results.
Between 23 and 27 August 2007, AGILE detected gamma-ray emission from PKS 1510-089 when this source was located
∼50◦off-axis, with an average flux of (270 ±65)×10−8photons cm−2s−1for photon energy above 100 MeV. In the following period,
28 August - 1 September, after a satellite re-pointing, AGILE detected the source at ∼35◦off-axis, with an average flux (E >100
MeV) of (195 ±30) ×10−8photons cm−2s−1. No emission was detected by Super-AGILE, with a 3-σupper limit of 45 mCrab in 200
ksec.
Conclusions.
The spectral energy distribution is modelled with a homogeneous one-zone synchrotron self Compton (SSC) emission
plus contributions by external photons: the SSC emission contributes primarily to the X-ray band, whereas the contribution of the IC
from the external disc and the broad line region match the hard gamma-ray spectrum observed.
Key words. gamma-rays: observations – mechanism: non-thermal – quasars: individual (PKS 1510-089)
1. Introduction
The radio source PKS 1510-089 was first identified optically as
a quasar with an ultraviolet excess, a visual magnitude of 16.5
(Bolton & Ekers 1966), and a redshift of z =0.361 measured
from its emission-line spectrum (Burbidge & Kinnan 1966).
PKS 1510-089 is a radio-loud highly polarized quasar (HPQ)
belonging to the class of the flat spectrum radio quasar (FSRQ)
in terms of its spectral energy distribution. Its radiative output is
dominated by the gamma-ray component, while its synchrotron
emission peaks around IR frequencies below a pronunced UV
bump, presumably due to the thermal emission from the accre-
tion disc (Malkan & Moore 1986, Pian & Treves 1993).
PKS 1510-089 has been extensively observed in X-rays by
EXOS AT (Singh, Rao & Vahia 1990, Sambruna et al. 1994),
GINGA (Lawson & Turner 1997), ROS AT (Siebert et al. 1998),
ASCA (Singh, Shrader & George 1997) and Chandra (Gambill
et al. 2003). The observed X-ray spectrum was very flat in the
2÷10 keV band (photon index Γ≃1.3), but steepened in
the ROS AT bandpass below 2 keV (Γ≃1.9). Observations by
BeppoS AX (Tavecchio et al. 2000) confirm the presence of a
soft X-ray excess below 1 keV. A similar soft excess has been
detected in other blazarssuch as 3C273, 3C279, and 3C454.3,
and the origin of the soft X-ray excess is still an open issue.
PKS 1510 was recently observed by S uzaku in August 2006
over approximately three days, and the campaign continued with
Swift monitoring over 18 days (Kataoka et al. 2007). Swift-XRT
observations reveal significant spectral evolution of the X-ray
emission on timescales of one week: the X-ray spectrum be-
comes harder as the source gets brighter.
Gamma-ray emission from PKS 1510-089 was detected by
the EGRET instrument on board CGRO with a integrated flux
above 100 MeV between (13 ±5) and (49 ±18) ×10−8photons
cm−2s−1and an energy spectrum modelled with a power law
with a photon index Γ = 2.47 ±0.21. In this Letter we present
the analysis of the AGILE data obtained during the PKS 1510-
089 observations from 23 August 2007 to 1 September 2007.
2. AGILE observation of PKS 1510-089
The AGILE scientific Instrument (Tavani et al. 2008) is very
compact and combines four active detectors yielding broad-band
coverage from hard X-rays to gamma rays.
The Gamma-Ray Imaging Detector (GRID) consists of a
combination of a silicon tracker (Prest et al. 2003, Barbiellini
et al. 2001), a non-imaging Cesium Iodide Mini-Calorimeter
(MCAL; Labanti et al. 2006) positioned under the silicon tracker
and sensitive in the 0.3 ÷100 MeV energy band, and a seg-
mented anticoincidence system (ACS) made of a plastic shield
which surrounds all active detectors (Perotti et al. 2006). A co-
aligned coded-mask hard X-ray imager (SuperAGILE; Costa et
al. 2001, Feroci et al. 2007) ensures coverage in the 18 ÷60 keV
energy band.
The GRID has a field of view of ∼2.5 sr, an angular resolu-
tion of 1.2◦at 400 MeV (68% cont. radius), an effective area of
∼500 cm2above 100 MeV, and an energy resolution ∆E/E∼1
at 400 MeV. The silicon tracker and the on-board trigger logic
are optimized for gamma-ray imaging in the 30 MeV ÷50 GeV
energy band (Argan et al. 2004).
The AGILE observations of the PKS 1510-089 were per-
formed from 23 August 2007 12:00 UT to 1 September 2007
Send offprint requests to: G. Pucella, e-mail:
gianluca.pucella@iasf-roma.inaf.it
Fig.1. Gaussian-smoothed count map (∼140◦×140◦) in
Galactic coordinates integrated over the observing period 2007
August 23 - August 27. The circles are located at the PKS
1510-089 and Vela Pulsar coordinates. Only photons with en-
ergy greater than 100 MeV have been included.
Fig.2. Gaussian-smoothed count map (∼140◦×140◦) in
Galactic coordinates integrated over the observing period 2007
August 28 - September 1. The circle is located at the PKS 1510-
089 coordinates. Only photons with energy greater than 100
MeV have been included.
12:00 UT, for a total of 84 hours of effective exposure time.
In the first period, between 23 and 27 August, the source was
located ∼50◦offthe AGILE pointing direction. In the second
period, between 28 August and 1 September, after a satellite re-
pointing, the source was located at ∼35◦off-axis.
3. Data reduction and analysis
Level–1 AGILE-GRID data were analysed using the AGILE
Standard Analysis Pipeline. The first step is to align all data
times to terrestrial time (TT), and it performs preliminary
calculations. In the second step, an ad-hoc implementation
of the Kalman Filter technique is used for track identifica-
tion and event-direction reconstruction in detector coordinates.
Subsequently, a quality flag is assigned to each GRID event:
(G), (P), (S), and (L), depending on whether it is recognised as a
confirmed gamma-ray event, a charged particle event, a single-
track event, or its nature is uncertain, respectively. The (L) event
class includes events typically affected by an order of magni-
tude higher particle contamination than (G). The single-track
G. Pucella et al.: AGILE detection of PKS 1510-089 3
Fig.3. a) AGILE-GRID gamma-ray light curve, with a 1-
day resolution, for the observation period 2007 August 28 -
September 1, for photons with E >100 MeV in units of 10−8
photons cm−2s−1. b) R-band optical light curve as observed by
the GASP of the WEBT for the observation period 2007 July 26
- September 11. c) GASP radio light curve (from UMRAO) at
14.5 GHz for the observation period 2007 July 24 - September
15.
(S) event class includes events for which only one track is re-
constructed in the two orthogonal views of the tracker. Then, an
AGILE log-file is created, containing all the information rele-
vant to computating the exposure and live time. The third step
is to create the AGILE event files, excluding events flagged as
particles. This step also reconstructs the event direction in sky
coordinates.
Once the above steps are completed, the AGILE Scientific
Analysis Package can be run. Counts, exposure, and Galactic
background gamma-ray maps are created with a bin-size of
0.5◦×0.5◦for photons with energy over 100 MeV. To reduce
the particle background contamination, we selected only events
flagged as confirmed gamma-ray events, and all events collected
during the South Atlantic Anomaly were rejected.
We also rejected all the gamma-ray events whose recon-
structed directions form angles with the satellite-Earth vector
smaller than 80◦, reducing the gamma-ray Earth albedo contam-
ination by excluding regions within ∼10◦from the Earth limb.
We ran the AGILE maximum likelihood procedure (ALIKE) on
the whole observing period, in order to obtain the average flux
in the gamma-ray band.
Fig.4. AGILE average gamma-ray spectrum of PKS 1510-089
for the observation period 28 August - 1 September 2007.
4. Results
Figure 1 shows a Gaussian-smoothed count map in Galactic co-
ordinates integrated over the observing period 2007 August 23 -
27 for photon energies over 100 MeV. In this period, AGILE de-
tected gamma-ray emission from a position consistent with the
quasar PKS 1510-089 at a significance level of 5.6-σas derived
from a maximum likelihood analysis using the PKS 1510-089
radio position (l=351.29◦,b=40.14◦). During this period
the source was located ∼50◦offthe AGILE pointing direc-
tion. Thanks to the large field of view of AGILE, during this
period, the satellite simultaneously also detected the gamma ac-
tivity of the source Vela Pulsar (l=263.55◦,b=−2.79◦), about
90 degrees from PKS 1510-089. In addition, the two sources
were characterised by an almost equal angular distance from the
AGILE pointing direction. This allowed us to obtain an estimate
of average flux of PKS 1510-089 through a direct calibration
with the flux of the Vela Pulsar. In this way, the average flux (E
>100 MeV) estimated for this first period was (270 ±65)×10−8
photons cm−2s−1. The reduced effective area and the conse-
quently reduced count statistics for these large off-axis angles
makes it difficult to create a light curve and an average energy
spectrum.
Figure 2 shows a Gaussian-smoothed count map in Galactic
coordinates integrated over the observing period 2007 August
27 – September 1 for photon energies higher than 100 MeV.
In this period the satellite detected the source at a significance
level of 10.6-σ. With the likelihood method, the best position
for the gamma-ray emission above 100 MeV is (l=350.89◦,b=
40.34◦), with an average flux (E >100 MeV) over the period of
(195±30)×10−8photons cm−2s−1. The AGILE 95% maximum-
likelihood contour level has a semi-major axis a=0.43◦and
semi-minor axis b=0.09◦. The overall AGILE error circle, tak-
ing both statistical and systematic effects into account, has a ra-
dius r=0.53◦. During this period the source was observed at 35◦
off-axis. In Fig. 3 the gamma-ray light curve for this period with
a 1-day resolution, the R-band optical light curve as observed by
the GASP of the WEBT for the observation period between 26
July and 11 September 2007, and the GASP radio light curve
(from UMRAO) at 14.5 GHz for the observation period between
24 July and 15 September 2007 1are reported. Figure 4 reports
the average energy spectrum for this second period, when only
three energy bins were taken into account for the fit: 100 ÷200
1The optical and radio data presented in this paper are stored in the
GASP-WEBT archive (http://www.oato.inaf.it/blazars/webt). For ques-
tions regarding their availability, please contact the WEBT President
Massimo Villata.
4 G. Pucella et al.: AGILE detection of PKS 1510-089
Fig.5. Spectral Energy Distribution of PKS 1510-089 for the GRID observation period 28 August - 1 September 2007 (blue dots),
including simultaneous GASP optical (red square) and radio (orange triangle) data and the 3-σSuper-AGILE upper limit. Non-
simultaneous historical data (from 1969 to 2007) taken from NASA Extragalactic Database (NED) and Kataoka et al. (2007) over
the entire electromagnetic spectrum are represented in dark grey and light grey,respectively.
MeV, 200 ÷400 MeV, 400 ÷1000 MeV. The energy bins 50 ÷
100 MeV and 1000 ÷3000 MeV have not be used in the spec-
tral analysis waiting for on-flight calibration finalization that will
include these two energy channels.
A simple power-law model can fit the data. We obtain a
photon index Γ = 1.98 ±0.27 with the weighted least squares
method.
Super-AGILE observed PKS 1510-089 for a total on-source
effective exposure time of 200 ks. The source was not detected
above 5-σby the Super-AGILE Iterative Removal Of Sources
(IROS) applied to the image, in the 20–60 keV energy range. A
3-σupper limit of 45 mCrab was obtained from the observed
count rate by a study of the background fluctuations at the posi-
tion of the source and a simulation of the source and background
contributions with IROS.
Finally, in Fig. 5 the spectral energy distribution is shown
for the GRID observation period 2007 August 28 - September
1, including simultaneous GASP optical and radio data and the
3-σSuper-AGILE upper limit. Also non-simultaneous historical
data over the entire electromagnetic spectrum are represented.
5. Discussion
During the AGILE observation period, the PKS 1510-089 opti-
cal flux appears to be decreasing in the range R∼15.5 ÷15.8,
following a bright state that reached at least R=15.0 (see Fig.
3). The contemporaneous gamma-ray flux decrease of about a
factor 3 suggests that the two flux variations may be correlated.
In agreement with Kataoka et al. (2007), in order to model the
spectral energy distribution we used a homogeneous one-zone
synchrotron self-Compton (SSC) model, plus the contribution
of external seed photons originating both from the accretion disc
and the broad line region (BLR). We obtained a representative fit
for the spectral energy distribution with input parameters similar
to those chosen by Kataoka et al. (2007). We consider a rela-
tivistic moving spherical blob of radius R =2.5 ×1016 cm and
an electron energy distribution described by a double power law:
ne(γ)=Kγ−1
b
(γ/γb)p1+(γ/γb)p2(1)
for electron Lorentz factor 40 < γ < 4×103with spectral indices
pre- and post-break p1=2.0 and p2=4.5, a normalization
factor K =80 cm−3and the break energy Lorentz factor γb=
400. We assumed a magnetic field B =3 Gauss and a Doppler
factor δ=9 for the blob.
In order to interpret our gamma-ray data, an accretion disc
characterised by a blackbody spectrum with a luminosity of
1046erg s−1at 0.1 pc from the blob is assumed as the source
of external target photons. The inverse Compton (IC) contribu-
tion from the disc is calculated up to the second order, but it is
not enough to account for the high gamma-ray state observed
by AGILE. The addition of the IC emission from a BLR, repre-
sented by a spherical layer extending between 0.1 pc and 0.4 pc
from the central black hole, reprocessing a 10% of the irradiating
continuum can explain the high state observed by AGILE com-
pared to the historical EGRET observations (see Fig. 5), and it
reflects on the different photon index obtained in the AGILE and
EGRET observations. In this model, the SSC emission primarily
contributes to the X-ray band, whereas the IC contribution from
the BLR can explain the observed hard gamma-ray spectrum.
Acknowledgements. The AGILE Mission is funded by the Italian Space Agency
(ASI) with scientific and programmatic participation by the Italian Institute of
Astrophysics (INAF) and the Italian Institute of Nuclear Physics (INFN). This
research has made use of the NASA/IPAC Extragalactic Database (NED) which
is operated by the Jet Propulsion Laboratory, California Institute of Technology,
under contract with the National Aereonautics and Space Administration.
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