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The first observation of optical pulsations from a soft gamma repeater: SGR 0501+4516
Abstract
We present high-speed optical photometry of the soft gamma repeater SGR
0501+4516, obtained with ULTRACAM on two consecutive nights approximately 4
months after the source was discovered via its gamma-ray bursts. We detect SGR
0501+4516 at a magnitude of i' = 24.4+/-0.1. We present the first measurement
of optical pulsations from an SGR, deriving a period of 5.7622+/-0.0003 s, in
excellent agreement with the X-ray spin period of the neutron star. We compare
the morphologies of the optical pulse profile with the X-ray and infrared pulse
profiles; we find that the optical, infrared and harder X-rays share similar
double-peaked morphologies, but the softer X-rays exhibit only a single-peaked
morphology, indicative of a different origin. The optical pulsations appear to
be in phase with the X-ray pulsations and exhibit a root-mean-square pulsed
fraction of 52+/-7%, approximately a factor of two greater than in the X-rays.
Our results find a natural explanation within the context of the magnetar model
for SGRs.
... However tical radiation from magnetars has yet to be perceived as short flashes accompanyin dio bursts. They have been observed either as flares lasting tens of seconds [97] or as tical pulsations [98]. The optical flux of the pulsating radiation from SGR 0501+4516 in i' filter was at the level of 24 m. ...
... However, optical radiation from magnetars has yet to be perceived as short flashes accompanying radio bursts. They have been observed either as flares lasting tens of seconds [97] or as optical pulsations [98]. The optical flux of the pulsating radiation from SGR 0501+4516 in the i' filter was at the level of 24 m. ...
This review considers synchronous and follow-up MASTER Global Robotic Net optical observations of high energy astrophysical phenomena such as fast radio bursts (FRB), gamma-ray bursts (including prompt optical emission polarization discovery), gravitational-wave events, detected by LIGO/VIRGO (including GW170817 and independent Kilonova discovery), high energy neutrino sources (including the detection of IC-170922A progenitor) and others. We report on the first large optical monitoring campaign of the closest at that moment radio burster FRB 180916.J0158+65 simultaneously with a radio burst. We obtained synchronous limits on the optical flux of the FRB 180916.J0158+65 and FRB 200428 (soft gamma repeater SGR 1935+2154)(The CHIME/FRB Collaboration, Nature 2020, 587) at 155093 MASTER images with the total exposure time equal to 2,705,058 s, i.e., 31.3 days. It follows from these synchronous limitations that the ratio of the energies released in the optical and radio ranges does not exceed 4 × 105. Our optical monitoring covered a total of 6 weeks. On 28 April 2020, MASTER automatically following up on a Swift alert began to observe the galactic soft gamma repeater SGR 1935+2154 experienced another flare. On the same day, radio telescopes detected a short radio burst FRB 200428 and MASTER-Tavrida telescope determined the best prompt optical limit of FRB/SGR 1935+2154. Our optical limit shows that X-ray and radio emissions are not explained by a single power-law spectrum. In the course of our observations, using special methods, we found a faint extended afterglow in the FRB 180916.J0158+65 direction associated with the extended emission of the host galaxy.
... Other than at γ -ray, X-ray and radio wavelengths, a few magnetars have also been observed in the optical and infrared (e.g. Hulleman, v an K erkwijk & K ulkarni 2004 ;Kosugi, Ogasa wara & Terada 2005 ;Camilo et al. 2007 ;Testa et al. 2008 ;Dhillon et al. 2011 ;Tendulkar, Cameron & Kulkarni 2012 ). Given their locations in the Galactic plane, high dust extinctions have restricted these observations, b ut optical/near -infrared (NIR) variability has been noted in a handful of cases. ...
... Ho we ver, the NIR and X-rays do not al w ays vary synchronously Testa et al. 2008 ;Lyman et al. 2022 ). The aforementioned variability has been reported for a handful of sources o v er month to year time-scales, but short time-scale ( ∼seconds) pulsations have also been seen (Kern & Martin 2002 ;Dhillon et al. 2011 ). ...
We report the discovery of six new magnetar counterpart candidates from deep near-infrared Hubble Space Telescope imaging. The new candidates are among a sample of nineteen magnetars for which we present HST data obtained between 2018–2020. We confirm the variability of previously established near-infrared counterparts, and newly identify candidates for PSR J1622-4950, Swift J1822.3-1606, CXOU J171405.7-381031, Swift J1833-0832, Swift J1834.9-0846 and AX J1818.8-1559 based on their proximity to X-ray localisations. The new candidates are compared with the existing counterpart population in terms of their colours, magnitudes, and near-infrared to X-ray spectral indices. We find two candidates for AX J1818 which are both consistent with previously established counterparts. The other new candidates are likely to be chance alignments, or otherwise have a different origin for their near-infrared emission not previously seen in magnetar counterparts. Further observations and studies of these candidates are needed to firmly establish their nature.
... Other than at -ray, X-ray and radio wavelengths, a few magnetars have also been observed in the optical and infrared (e.g. Hulleman et al. 2004;Kosugi et al. 2005;Camilo et al. 2007;Testa et al. 2008;Dhillon et al. 2011;Tendulkar et al. 2012). Given their locations in the Galactic plane, high dust extinctions have restricted these observations, but optical/near-infrared (NIR) variability has been noted in a handful of cases. ...
... However, the NIR and X-rays do not always vary synchronously Testa et al. 2008;Lyman et al. 2021). The aforementioned variability has been reported for a handful of sources over month to year time-scales, but short time-scale (∼seconds) pulsations have also been seen (Kern & Martin 2002;Dhillon et al. 2011). ...
We report the discovery of six new magnetar counterpart candidates from deep near-infrared Hubble Space Telescope imaging. The new candidates are among a sample of nineteen magnetars for which we present HST data obtained between 2018-2020. We confirm the variability of previously established near-infrared counterparts, and newly identify candidates for PSRJ1622-4950, SwiftJ1822.3-1606, CXOUJ171405.7-381031, SwiftJ1833-0832, SwiftJ1834.9-0846 and AXJ1818.8-1559 based on their proximity to X-ray localisations. The new candidates are compared with the existing counterpart population in terms of their colours, magnitudes, and near-infrared to X-ray spectral indices. We find two candidates for AXJ1818.8-1559 which are both consistent with previously established counterparts. The other new candidates are likely to be chance alignments, or otherwise have a different origin for their near-infrared emission not previously seen in magnetar counterparts. Further observations and studies of these candidates are needed to firmly establish their nature.
... Optical and NIR observations were often carried out once such outbursts/flares were identified by X-ray space telescopes. Among 21 confirmed magnetars (Mcgill SGR/AXP Online Catalog; Olausen and Kaspi 2013), three other magnetars were confirmed with NIR counterparts during their outbursts: SGR 0501+4516 (Dhillon et al., 2011), SGR 1806−20 (Israel et al., 2005), and XTE J1810−197 (Camilo et al., 2007;Testa et al., 2008). The NIR detections benefited from their emission brightening during outbursts, and for SGR 0501+4516 a possible 20% pulsed emission at K band was detected (Dhillon et al., 2011). ...
... Among 21 confirmed magnetars (Mcgill SGR/AXP Online Catalog; Olausen and Kaspi 2013), three other magnetars were confirmed with NIR counterparts during their outbursts: SGR 0501+4516 (Dhillon et al., 2011), SGR 1806−20 (Israel et al., 2005), and XTE J1810−197 (Camilo et al., 2007;Testa et al., 2008). The NIR detections benefited from their emission brightening during outbursts, and for SGR 0501+4516 a possible 20% pulsed emission at K band was detected (Dhillon et al., 2011). The nature of the NIR emission is not clearly understood. ...
Different pieces of observational evidence suggest the existence of disks
around isolated neutron stars. Such disks could be formed from supernova
fallback when neutron stars are born in core-collapse supernova explosions.
Efforts have been made to search for disks around different classes of pulsars,
which include millisecond pulsars, young neutron star classes (magnetars,
central compact objects, and X-ray dim isolated neutron stars), and regular
radio pulsars. We review the main results from observations at wavelengths of
from optical to sub-millimeter/millimeter.
... A p = 1.6% rejection of the null-hypothesis of no variability for the latest, and brightest, epoch does perhaps offering some marginal indication that warrants further investigation. Although short period variability in SGRs on the timescales of their magnetars' rotation periods (seconds) have been found (e.g., Kern & Martin 2002;Dhillon et al. 2011), the origin of any minutes-hour long time-scale variability would be less obvious. Additional observations while the source is brighter (which enable more precise pho-tometry), have a better chance to rule on the presence of variability of the source over these timescales. ...
We present deep Hubble Space Telescope near-infrared (NIR) observations of the magnetar SGR 1935+2154 from June 2021, approximately 6 years after the first HST observations, a year after the discovery of fast radio burst like emission from the source, and in a period of exceptional high frequency activity. Although not directly taken during a bursting period the counterpart is a factor of ~1.5 to 2.5 brighter than seen at previous epochs with F140W(AB) = $24.65\pm0.02$ mag. We do not detect significant variations of the NIR counterpart within the course of any one orbit (i.e. on minutes-hour timescales), and contemporaneous X-ray observations show SGR 1935+2154 to be at the quiescent level. With a time baseline of 6 years from the first identification of the counter-part we place stringent limits on the proper motion of the source, with a measured proper motion of ${\mu} = 3.1\pm1.5$ mas/yr. The direction of proper motion indicates an origin of SGR 1935+2154 very close to the geometric centre of SNR G57.2+08, further strengthening their association. At an adopted distance of $6.6\pm0.7$ kpc, the corresponding tangential space velocity is ${\nu_T} = 97\pm48$ km/s (corrected for differential Galactic rotation and peculiar Solar motion), although its formal statistical determination may be compromised owing to few epochs of observation. The current velocity estimate places it at the low end of the kick distribution for pulsars, and makes it among the lowest known magnetar kicks. When collating the few-magnetar kick constraints available, we find full consistency between the magnetar kick distribution and the much larger pulsar kick sample
We present deep Hubble Space Telescope (HST) near-infrared (NIR) observations of the magnetar SGR 1935+2154 from 2021 June, approximately 6 yr after the first HST observations, a year after the discovery of fast-radio-burst-like emission from the source, and in a period of exceptional high-frequency activity. Although not directly taken during a bursting period the counterpart is a factor of ∼1.5–2.5 brighter than seen at previous epochs with F140W(AB) = 24.65 ± 0.02 mag. We do not detect significant variations of the NIR counterpart within the course of any one orbit (i.e., on minutes to hour timescales), and contemporaneous X-ray observations show SGR 1935+2154 to be at the quiescent level. With a time baseline of 6 yr from the first identification of the counterpart we place stringent limits on the proper motion (PM) of the source, with a measured PM of μ = 3.1 ± 1.5 mas yr ⁻¹ . The direction of PM indicates an origin of SGR 1935+2154 very close to the geometric center of SNR G57.2+08, further strengthening their association. At an adopted distance of 6.6 ± 0.7 kpc, the corresponding tangential space velocity is ν T = 97 ± 48 km s ⁻¹ (corrected for differential Galactic rotation and peculiar solar motion), although its formal statistical determination may be compromised owing to few epochs of observation. The current velocity estimate places it at the low end of the kick distribution for pulsars, and makes it among the lowest known magnetar kicks. When collating the few-magnetar kick constraints available, we find full consistency between the magnetar kick distribution and the much larger pulsar kick sample.
We currently know about 30 magnetars: seemingly isolated neutron stars whose properties can be (in part) comprehended only acknowledging that they are endowed with magnetic fields of complex morphology and exceptional intensity—at least in some components of the field structure. Although magnetars represent only a small percentage of the known isolated neutron stars, there are almost certainly many more of them, since most magnetars were discovered in transitory phases called outbursts, during which they are particularly noticeable. In outburst, in fact, a magnetar can be brighter in X-rays by orders of magnitude and usually emit powerful bursts of hard-X/soft-gamma-ray photons that can be detected almost everywhere in the Galaxy with all-sky monitors such as those on board the Fermi satellite or the Neil Gehrels Swift Observatory. Magnetars command great attention because the large progress that has been made in their understanding is proving fundamental to fathom the whole population of isolated neutron stars, and because, due to their extreme properties, they are relevant for a vast range of different astrophysical topics, from the study of gamma-ray bursts and superluminous supernovae, to ultraluminous X-ray sources, fast radio bursts, and even to sources of gravitational waves. Several excellent reviews with different focuses were published on magnetars in the last few years: among others, Israel and Dall’Osso, Bursts and flares from highly magnetic pulsars, in Proceedings of the First Session of the Sant Cugat Forum on Astrophysics High-Energy Emission from Pulsars and Their Systems, ed. by D.F. Torres, N. Rea. Astrophysics and Space Science Proceedings (Springer, Heidelberg, 2011), pp. 279–298, Rea and Esposito, Magnetar outbursts: an observational review, in High-Energy Emission from Pulsars and Their Systems. Proceedings of the First Session of the Sant Cugat Forum on Astrophysics, ed. by D.F. Torres, N. Rea. Astrophysics and Space Science Proceedings (Springer, Heidelberg, 2011), pp. 247–273, Turolla and Esposito Int J Mod Phys D 22:1330024-163, 2013, Mereghetti, et al. Space Sci Rev 191:315–338, 2015, Turolla et al., 78:116901, 2015; Kaspi and Beloborodov Annu Rev Astron Astrophys 55:261–301, 2017. Here, we quickly recall the history of these sources and travel through the main observational facts, trying to touch some recent and sometimes little-discussed ramifications of magnetars.
Young and rotation-powered neutron stars (NSs) are commonly observed as rapidly-spinning pulsars. They dissipate their rotational energy by emitting pulsar wind with electromagnetic radiation and spin down at a steady rate, according to the simple steadily-rotating magnetic dipole model. In reality, however, multiwavelength observations of radiation from the NS surface and magnetosphere have revealed that the evolution and properties of NSs are highly diverse, often dubbed as 'NS zoo'. In particular, many of young and highly magnetized NSs show a high degree of activities, such as sporadic electromagnetic outbursts and irregular changes in pulse arrival times. Importantly, their magnetic field, which are the strongest in the universe, makes them ideal laboratories for fundamental physics. A class of highly-magnetized isolated NSs is empirically divided into several subclasses. In a broad classification, they are, in the order of the magnetic field strength (B) from the highest, 'magnetars' (historically recognized as soft gamma-ray repeaters and/or anomalous x-ray pulsars), 'high-B pulsars', and (nearby) x-ray isolated NSs. This article presents an introductory review for non-astrophysicists about the observational properties of highly-magnetized NSs, and their implications. The observed dynamic nature of NSs must be interpreted in conjunction with transient magnetic activities triggered during magnetic-energy dissipation process. In particular, we focus on how the five fundamental quantities of NSs, i.e. mass, radius, spin period, surface temperature, and magnetic fields, as observed with modern instruments, change with evolution of, and vary depending on the class of, the NSs. They are the foundation for a future unified theory of NSs.
Magnetars are neutron stars in which a strong magnetic field is the main energy source. About two dozens of magnetars, plus several candidates, are currently known in our Galaxy and in the Magellanic Clouds. They appear as highly variable X-ray sources and, in some cases, also as radio and/or optical pulsars. Their spin periods (2–12 s) and spin-down rates (\(\sim10^{-13}\mbox{--}10^{-10}~\mbox{s}\,\mbox{s}^{-1}\)) indicate external dipole fields of \(\sim10^{13-15}\) G, and there is evidence that even stronger magnetic fields are present inside the star and in non-dipolar magnetospheric components. Here we review the observed properties of the persistent emission from magnetars, discuss the main models proposed to explain the origin of their magnetic field and present recent developments in the study of their evolution and connection with other classes of neutron stars.
Magnetars are young and highly magnetized neutron stars which display a wide array of X-ray activity including short bursts, large outbursts, giant flares and quasi-periodic oscillations, often coupled with interesting timing behavior including enhanced spin-down, glitches and anti-glitches. The bulk of this activity is explained by the evolution and decay of an ultrastrong magnetic field, stressing and breaking the neutron star crust, which in turn drives twists of the external magnetosphere and powerful magnetospheric currents. The population of detected magnetars has grown to about 30 objects and shows unambiguous phenomenological connection with very highly magnetized radio pulsars. Recent progress in magnetar theory includes explanation of the hard X-ray component in the magnetar spectrum and development of surface heating models, explaining the sources' remarkable radiative output.
UKIRT is a powerful facility for GRB follow-up. The infrared capability allows us to search for dusty and distant afterglows and the integrated observing and data reduction systems are well suited to rapid observation and pipeline processing of results. UKIRT has operated a GRB target-of-opportunity programme for a number of years, which has localised and monitored many GRB afterglows; has provided critical observations of GRB-like phenomena, such as the SGR 0501+4516; and recently made the discovery of the afterglow of GRB 090423 at a record-breaking redshift of z = 8.2.
Magnetars are the strongest magnets in the present universe and the
combination of extreme magnetic field, gravity and density makes them unique
laboratories to probe current physical theories (from quantum electrodynamics
to general relativity) in the strong field limit. Magnetars are observed as
peculiar, burst--active X-ray pulsars, the Anomalous X-ray Pulsars (AXPs) and
the Soft Gamma Repeaters (SGRs); the latter emitted also three "giant flares,"
extremely powerful events during which luminosities can reach up to 10^47 erg/s
for about one second. The last five years have witnessed an explosion in
magnetar research which has led, among other things, to the discovery of
transient, or "outbursting," and "low-field" magnetars. Substantial progress
has been made also on the theoretical side. Quite detailed models for
explaining the magnetars' persistent X-ray emission, the properties of the
bursts, the flux evolution in transient sources have been developed and
confronted with observations. New insight on neutron star asteroseismology has
been gained through improved models of magnetar oscillations. The long-debated
issue of magnetic field decay in neutron stars has been addressed, and its
importance recognized in relation to the evolution of magnetars and to the
links among magnetars and other families of isolated neutron stars. The aim of
this paper is to present a comprehensive overview in which the observational
results are discussed in the light of the most up-to-date theoretical models
and their implications. This addresses not only the particular case of magnetar
sources, but the more fundamental issue of how physics in strong magnetic
fields can be constrained by the observations of these unique sources.
Magnetars are neutron stars in which a strong magnetic field is the main
energy source. About two dozens of magnetars, plus several candidates, are
currently known in our Galaxy and in the Magellanic Clouds. They appear as
highly variable X-ray sources and, in some cases, also as radio and/or optical
pulsars. Their spin periods (2-12 s) and spin-down rates (~10^{-13}-10^{-10}
s/s) indicate external dipole fields of ~10^{13-15} G, and there is evidence
that even stronger magnetic fields are present inside the star and in
non-dipolar magnetospheric components. Here we review the observed properties
of the persistent emission from magnetars, discuss the main models proposed to
explain the origin of their magnetic field and present recent developments in
the study of their evolution and connection with other classes of neutron
stars.
We have analysed the long-term evolution and the X-ray outburst light curve of SGR 0501+4516 in the fallback disc model. We
have shown that the X-ray luminosity, period and period derivative of this typical soft gamma repeater can be achieved by
a neutron star with a large range of initial disc masses provided that the source has a magnetic dipole field of ∼1.4 × 1012 G on the pole of the star. At present, the star is accreting matter from the disc, which has an age ∼3 × 104 yr, and will remain in the accretion phase until t ∼2–5 × 105 yr depending on the initial disc mass. With its current rotational rate, this source is not expected to give pulsed radio
emission even if the accretion on to the star is hindered by some mechanism. The X-ray enhancement light curve of SGR 0501+4516
can be accounted for by the same model applied earlier to the X-ray enhancement light curves of other anomalous X-ray pulsars/soft
gamma repeaters with the same basic disc parameters. We have further shown that the optical/infrared data of SGR 0501+4516
are in good agreement with the emission from an irradiated fallback disc with the properties consistent with our long-term
evolution model.
We report on the quiescent state of the soft gamma repeater SGR 0501+4516 observed by XMM–Newton on 2009 August 30. The source exhibits an absorbed flux ∼75 times lower than that measured at the peak of the 2008 outburst,
and a rather soft spectrum, with the same value of the blackbody temperature observed with ROSAT back in 1992. This new observation is put into the context of all existing X-ray data since its discovery in 2008 August,
allowing us to complete the study of the timing and spectral evolution of the source from outburst until its quiescent state.
The set of deep XMM–Newton observations performed during the few years time-scale of its outburst allows us to monitor the spectral characteristics
of this magnetar as a function of its rotational period, and their evolution along these years. After the first ∼10 d, the
initially hot and bright surface spot progressively cooled down during the decay. We discuss the behaviour of this magnetar
in the context of its simulated secular evolution, inferring a plausible dipolar field at birth of 3 × 1014 G, and a current (magnetothermal) age of ∼10 kyr.
We present a catalog of the 26 currently known magnetars and magnetar candidates. We tabulate astrometric and timing data for all catalog sources, as well as their observed radiative properties, particularly the spectral parameters of the quiescent X-ray emission. We show histograms of the spatial and timing properties of the magnetars, comparing them with the known pulsar population, and we investigate and plot possible correlations between their timing, X-ray, and multiwavelength properties. We find the scale height of magnetars to be in the range of 20-31 pc, assuming they are exponentially distributed. This range is smaller than that measured for OB stars, providing evidence that magnetars are born from the most massive O stars. From the same fits, we find that the Sun lies ~13-22 pc above the Galactic plane, consistent with previous measurements. We confirm previously identified correlations between quiescent X-ray luminosity, L
X, and magnetic field, B, as well as X-ray spectral power-law indexes, Γ and B, and show evidence for an excluded region in a plot of L
X versus Γ. We also present an updated kT versus characteristic age plot, showing that magnetars and high-B radio pulsars are hotter than lower-B neutron stars of similar age. Finally, we observe a striking difference between magnetars detected in the hard X-ray and radio bands; there is a clear correlation between the hard and soft X-ray fluxes, whereas the radio-detected magnetars all have low, soft X-ray flux, suggesting, if anything, that the two bands are anticorrelated.
The high-energy sources known as anomalous X-ray pulsars (AXPs) and soft
gamma-ray repeaters (SGRs) are well explained as magnetars: isolated neutron
stars powered by their own magnetic energy. After explaining why it is
generally believed that the traditional energy sources at work in other neutron
stars (accretion, rotation, residual heat) cannot power the emission of
AXPs/SGRs, I review the observational properties of the twenty AXPs/SGRs
currently known and describe the main features of the magnetar model. In the
last part of this review I discuss the recent discovery of magnetars with low
external dipole field and some of the relations between AXPs/SGRs and other
classes of isolated neutron stars.
We report on the long-term X-ray monitoring with Swift, RXTE, Suzaku, Chandra, and XMM-Newton of the outburst of the newly discovered magnetar Swift J1822.3-1606 (SGR 1822-1606), from the first observations soon after the detection of the short X-ray bursts which led to its discovery, through the first stages of its outburst decay (covering the time span from 2011 July until the end of 2012 April). We also report on archival ROSAT observations which detected the source during its likely quiescent state, and on upper limits on Swift J1822.3-1606's radio-pulsed and optical emission during outburst, with the Green Bank Telescope and the Gran Telescopio Canarias, respectively. Our X-ray timing analysis finds the source rotating with a period of P = 8.43772016(2) s and a period derivative \dot{P}=8.3(2)\times 10^{-14} s s-1, which implies an inferred dipolar surface magnetic field of B ~= 2.7 × 10^13 G at the equator. This measurement makes Swift J1822.3-1606 the second lowest magnetic field magnetar (after SGR 0418+5729). Following the flux and spectral evolution from the beginning of the outburst, we find that the flux decreased by about an order of magnitude, with a subtle softening of the spectrum, both typical of the outburst decay of magnetars. By modeling the secular thermal evolution of Swift J1822.3-1606, we find that the observed timing properties of the source, as well as its quiescent X-ray luminosity, can be reproduced if it was born with a poloidal and crustal toroidal fields of Bp ~ 1.5 × 10^14 G and B tor ~ 7 × 10^14 G, respectively, and if its current age is ~550 kyr.
We report broad-band Hubble Space Telescope imaging of the field of soft
gamma-ray repeater SGR 0418+5729 with ACS/WFC and WFC3/IR. Observing in two
wide filters F606W and F110W, we find no counterpart within the positional
error circle derived from Chandra observations, to limiting magnitudes
mF606W>28.6, mF110W>27.4 (Vega system), equivalent to reddening-corrected
luminosity limits LF606W<5e28, LF110W<6e28 erg s-1 for a distance d=2 kpc, at
3sig confidence. This, in turn, imposes lower limits on the contemporaneous
X-ray/optical flux ratio of 1100 and X-ray/near-infra-red flux ratio of 1000.
We derive an upper limit on the temperature and/or size of any fall-back disk
around the magnetar. We also compare the detection limits with observations of
other magnetars.
The sky region including the Chandra position of SGR 1806-20 was
monitored in the IR band during 2004, following its increased high
energy bursting activity. Observations were performed using NAOS-CONICA,
the adaptive optics IR camera mounted on Yepun VLT, which provided
images of unprecedented quality (FWHM better than 0.1 arcsec). After the
2004 December 27th giant flare, the source position has been nailed by
VLA observations of its radio counterpart, reducing the positional
uncertainty to 0.04 arcsec. Using IR data from our monitoring campaign,
we discovered the likely IR counterpart to SGR 1806-20 based on
positional coincidence with the Chandra and VLA uncertainty regions and
flux variability of a factor of about 2 correlated with that at higher
energies. We compare our findings with other isolated neutron star
classes thought to be related, at some level, with SGRs.
We present optical data which suggests that G", the optical counterpart of the gamma -ray pulsar Geminga, possibly pulses in B with a period of 0.237 seconds. The similarity between the optical pulse shape and the gamma -ray light curve indicates that a large fraction of the optical emission is non-thermal in origin - contrary to recent suggestions based upon the total optical flux. The derived magnitude of the pulsed emission is m_B = 26.0 +/- 0.4. Whilst it is not possible to give an accurate figure for the pulsed fraction (due to variations in the sky background) we can give an 1 sigma upper limit of m_B ~ 27 for the unpulsed fraction. Based on Observations taken at ESO, La Silla, Chile and SAO, Nizhnij Arhyz, Russia
A radiative model for the soft gamma repeaters and the energetic 1979
March 5 burst is presented. We identify the sources of these bursts with
neutron stars the external magnetic fields of which are much stronger
than those of ordinary pulsars. Several independent arguments point to a
neutron star with B_dipole~5x10^14 G as the source of the March 5 event.
A very strong field can (i) spin down the star to an 8-s period in the
~10^4-yr age of the surrounding supernova remnant N49; (ii) provide
enough energy for the March 5 event; (iii) undergo a large-scale
interchange instability the growth time of which is comparable to the
~0.2-s width of the initial hard transient phase of the March 5 event;
(iv) confine the energy that was radiated in the soft tail of that
burst; (v) reduce the Compton scattering cross-section sufficiently to
generate a radiative flux that is ~10^4 times the (non-magnetic)
Eddington flux; (vi) decay significantly in ~10^4-10^5 yr, as is
required to explain the activity of soft gamma repeater sources on this
time-scale; and (vii) power the quiescent X-ray emission L_X~7x10^35 erg
s^-1 observed by Einstein and ROSAT as it diffuses the stellar interior.
We propose that the 1979 March 5 event was triggered by a large-scale
reconnection/interchange instability of the stellar magnetic field, and
the soft repeat bursts by cracking of the crust. The hard initial spike
of the March 5 event is identified with an expanding pair fireball, and
the soft tail of that burst, together with the short, soft repeat
bursts, with a pair plasma trapped in the stellar magnetosphere. We
construct a detailed radiative model that describes the cooling of such
a plasma. The opacity is dominated by the electron-baryon contaminant in
a cold surface layer, and the plasma releases energy as the edge of the
pair-dominated region propagates inward, in a cooling wave. The rate at
which the plasma volume contracts is limited either by the rate of
advection of heat toward the stellar surface (where the field is
strongest and the scattering opacity weakest), or by ablation of ions
and electrons from the stellar surface. The effective temperature of the
surface radiation depends only on the surface magnetic field strength in
the regime where the radiative flux is limited by ablation from the
neutron star surface (which is the regime of interest in the March 5
event), and otherwise is weakly dependent on the plasma energy density.
We argue that the deposition of equivalent energy in a much weaker
magnetic field (characteristic of ordinary pulsars) necessarily
generates a very high scattering depth which chokes off the radiative
flow on the observed ~0.1-s time-scale of soft gamma repeater bursts.
Indeed, we suggest that the same basic magnetospheric emission mechanism
operates at lower field strengths (B~10^12 G) in Type II X-ray bursts,
which have much lower luminosities than soft gamma repeater bursts.
Important magnetic radiative effects include the suppression of Compton
scattering in the extraordinary polarization mode, and stimulated photon
splitting. We derive the Boltzmann equations for the photon occupation
number which describe simulated photon splitting, as well as photon
merging. We show that the net splitting rate vanishes in thermal
equilibrium. Radiative diffusion occurs primarily in the E-mode,
although rapid scattering of the O-mode ensures convergence of the
photon distribution function to a Bose-Einstein form. This allows us to
write down diffusion equations for the photon energy flux and number
flux as linear superpositions of gradients in the temperature and
chemical potential. The transition from a Planck to a Bose-Einstein
spectrum occurs at T~10 keV. Photon splitting at higher temperatures can
impede free-streaming of photons across the magnetic field lines, but
splitting of high-energy photons is impeded by the inverse process of
photon merging. We demonstrate that only a small fraction of the pair
bubble energy can be conducted into the crust during the lifetime of the
burst. We discuss the radiative ablation of matter from the heated
stellar surface after the magnetospheric pair plasma is dissipated. The
ensuing surface afterglow is typically ~1 per cent of the burst
luminosity. Direct pair neutrino cooling of the plasma is shown to be
unimportant for soft gamma repeater bursts, but may help to determine
the light curve of the March 5 event. Finally, we make a critical
comparison between this model and models in which the soft gamma
repeater bursts are triggered by accretion and/or involve surface
cooling.
We present the 158 standard stars that define the u'g'r'i'z' photometric system. These stars form the basis for the photometric calibration of the Sloan Digital Sky Survey. The defining instrument system and filters, the observing process, the reduction techniques, and the software used to create the stellar network are all described. We briefly discuss the history of the star selection process, the derivation of a set of transformation equations for the UBVRCIC system, and plans for future work.
Soft gamma repeaters and anomalous x-ray pulsars form a rapidly increasing
group of x-ray sources exhibiting sporadic emission of short bursts. They are
believed to be magnetars, i.e. neutron stars powered by extreme magnetic
fields, B~10^{14}-10^{15} Gauss. We report on a soft gamma repeater with low
magnetic field, SGR 0418+5729, recently detected after it emitted bursts
similar to those of magnetars. X-ray observations show that its dipolar
magnetic field cannot be greater than 7.5x10^{12} Gauss, well in the range of
ordinary radio pulsars, implying that a high surface dipolar magnetic field is
not necessarily required for magnetar-like activity. The magnetar population
may thus include objects with a wider range of B-field strengths, ages and
evolutionary stages than observed so far.
There is a general consensus about the fact that the magnetar scenario
provides a convincing explanation for several of the observed properties of the
Anomalous X-ray Pulsars and the Soft Gamma Repeaters. However, the origin of
the emission observed at low energies is still an open issue. We present a
quantitative model for the emission in the optical/infrared band produced by
curvature radiation from magnetospheric charges, and compare results with
current magnetars observations.
. We report on near-infrared (IR) observations of the three anomalous X-ray pulsars XTE J1810-197, 1RXS J1708-4009, 1E 1841-045 and the soft gamma-ray repeater SGR 1900+14, taken with the ESO-VLT, the Gemini, and the CFHT telescopes. . This work is aimed at identifying and/or confirming the IR counterparts of these magnetars, as well as at measuring their possible IR variability. . In order to perform photometry of objects as faint as Ks~20, we have used data taken with the largest telescopes, equipped with the most advanced IR detectors and in most of the cases with Adaptive Optics devices. The latter are critical to achieve the sharp spatial accuracy required to pinpoint faint objects in crowded fields. . We confirm with high confidence the identification of the IR counterpart to XTE J1810-197, and its IR variability. For 1E 1841-045 and SGR 1900+14 we propose two candidate IR counterparts based on the detection of IR variability. For 1RXS J1708-4009 we show that none of the potential counterparts within the source X-ray error circle can be yet convincingly associated with this AXP. . The IR variability of the AXP XTE J1810-197 does not follow the same monotonic decrease of its post-outburst X-ray emission. Instead, the IR variability appears more similar to the one observed in radio band, although simultaneous IR and radio observations are crucial to draw any conclusion in this respect. For 1E 1841-045 and SGR 1900+14, follow-up observations are needed to confirm our proposed candidates with higher confidence. Comment: 9 pages, 10 figures; accepted by A&A (high resolution images at http://staff.science.uva.nl/~nrea/NIR_AXP_testa2007.pdf)
We triggered our SGR ToO Program with RXTE following the Swift detection
of a burst from the new Soft Gamma Repeater SGR 0501+4516 (Barthelmy et
al. 2008, GCN # 8113). A 600 s RXTE observation started on 2008 August
22, 16:39:09 UT. During this pointing we detected one short SGR burst-
like event in the PCA data (2-60 keV), confirming that the new source
was in the field of view. We searched for a spin period for the new SGR
in the 2.5-13.5 keV band PCA event mode data and detected a coherent
signal (barycenter corrected) at 0.17334 Hz, corresponding to a spin
period of 5.769 ± 0.004 s.
The Swift satellite has been monitoring SGR0501+4516 (ATel #1676, #1677, #1678, #1682, #1683, #1688, #1691, #1692 and #1824) since the first burst detected by the BAT on board Swift on 22nd August 2008. After about two months since the BAT event, we detect an additional component with respect to the phase-coherent timing solution previously reported which included only the period and its first time derivative (ATel #1692; P=5.7620699(4)s and Pdot=5(1) x 10^-12 s/s; 1 sigma uncertainties).
The Lomb-Scargle method performs spectral analysis on unevenly sampled data and is known to be a powerful way to find, and test the significance of, weak periodic signals. The method has previously been thought to be 'slow', requiring of order 10(2)N(2) operations to analyze N data points. We show that Fast Fourier Transforms (FFTs) can be used in a novel way to make the computation of order 10(2)N log N. Despite its use of the FFT, the algorithm is in no way equivalent to conventional FFT periodogram analysis.
We present high-speed optical photometry of the anomalous X-ray pulsar 1E 1048.1-5937 obtained with ULTRACAM on the 8.2-m
Very Large Telescope in 2007 June. We detect 1E 1048.1-5937 at a magnitude of i′= 25.3 ± 0.2, consistent with the values found by Wang et al. and hence confirming their conclusion that the source was approximately
1 mag brighter than in 2003–06 due to an on-going X-ray flare that started in 2007 March. The increased source brightness
enabled us to detect optical pulsations with an identical period (6.458 s) to the X-ray pulsations. The root-mean-square (rms)
pulsed fraction in our data is 21 ± 7 per cent, approximately the same as the 2–10 keV X-ray rms pulsed fraction. The optical
and X-ray pulse profiles show similar morphologies and appear to be approximately in phase with each other, the latter lagging
the former by only 0.06 ± 0.02 cycles. The optical pulsations in 1E 1048.1-5937 are very similar in nature to those observed
in 4U 0142+61. The implications of our observations for models of anomalous X-ray pulsars are discussed.
We consider the structure of neutron star magnetospheres threaded by large-scale electrical currents and the effect of resonant Compton scattering by the charge carriers (both electrons and ions) on the emergent X-ray spectra and pulse profiles. In the magnetar model for the soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs), these currents are maintained by magnetic stresses acting deep inside the star, which generate both sudden disruptions (SGR outbursts) and more gradual plastic deformations of the rigid crust. We construct self-similar force-free equilibria of the current-carrying magnetosphere with a power-law dependence of magnetic field on radius, B~r-(2+p), and show that a large-scale twist of field lines softens the radial dependence of the magnetic field to p
We continued monitoring the new Soft Gamma Repeater SGR 0501+4516
(Barthelmy et al. 2008, GCN # 8113, Palmer et al. 2008, ATel # 1678)
using our SGR ToO Program with RXTE. We acquired a total exposure of 40
ks in 13 RXTE pointings performed over a time span of ~7.5 days. The
time spacing between each pointing ranged between 0.1 - 0.9 d. We detect
numerous (between 1 to 50) short bursts in observations before 2008
August 28.
The magnetosphere is described for a system consisting of a spinning
collapsed star connected by an aligned magnetic dipole field (B) to a
surrounding idealized coaxial, nonviscous cool Keplerian disk. When the
inner part of the conducting disk rotates more rapidly than the star,
the magnetosphere is charge-separated. A part of the magnetosphere will
corotate either with the star or with that part of the disk to which it
is linked by the magnetic field. Separating such differently rotating
regions are "gaps" empty of plasma. The magnetic field, including that
part which penetrates through the disk, remains everywhere
time-independent. Except in the gap and in the disk, the electric field
component is E . B ≅ 0. The dynamo electromotive force
from the (differently) rotating star and disk connected by B is balanced
by the potential drop along B in the gap. The model gap potential drop
is about 1016 V for disk and star parameters of a low-mass
X-ray binary (LMXB) and about 1015 V for those of typical
X-ray pulsar binaries. If very hot plasma from the star or the disk does
not quench the accelerator, accretion-powered X-rays which traverse the
gap limit its accelerating electric field by the e±
pair-production processes which they support. In the resulting steady
state a constant current flows through the accelerator which gives a
twist to, but does not continually stretch, the magnetic field through
the disk. The steady state gap potential drop can accelerate proton
(ions) toward the disk with enough energy so that interactions in the
disk can give TeV γ-rays. For LMXB parameters the model gives
strong e± production and hard X-ray and MeV
γ-ray emission with total power near that of accretion-powered
X-rays. With X-ray pulsar binary parameters the model total y-ray power
is typically about 10-2 times the full accretion X-ray power.
Possible application to disks around magnetic white dwarfs is
considered.
This paper is primarily an investigation of whether the `optimal
extraction' techniques used in CCD spectroscopy can be applied to
imaging photometry. It is found that using such techniques provides a
gain of around 10 per cent in signal-to-noise ratio over normal aperture
photometry. Formally, it is shown to be equivalent to profile fitting,
but offers advantages of robust error estimation, freedom from bias
introduced by mis-estimating the point spread function, and convenience.
In addition some other techniques are presented, which can be applied to
profile fitting, aperture photometry and the `optimal' photometry. Code
implementing these algorithms is available at
http://www.astro.keele.ac.uk/~timn/.
We derive a period determination technique that is well suited to the
case of nonsinusoidal time variation covered by only a few irregularly
spaced observations. A detailed statistical analysis allows comparison
with other techniques and indicates the optimum choice of parameters for
a given problem. Application to the double-mode Cepheid BK Cen
demonstrates the applicability of these methods to difficult cases.
Using 49 photoelectric points, we obtain the two primary oscillatory
components as well as the principal mode-interaction term; the derived
periods are in agreement with previous estimates.
We report here on the outburst onset and evolution of the new soft gamma-ray repeater SGR 0501+4516. We monitored the new SGR with XMM–Newton starting on 2008 August 23, 1 day after the source became burst active, and continuing with four more observations in the following month, with the last one on 2008 September 30. Combining the data with the Swift X-ray telescope (Swift–XRT) and Suzaku data, we modelled the outburst decay over a 3-month period, and we found that the source flux decreased exponentially with a time-scale of tc= 23.8 d. In the first XMM–Newton observation, a large number of short X-ray bursts were observed, the rate of which decayed drastically in the following observations. We found large changes in the spectral and timing behaviour of the source during the first month of the outburst decay, with softening emission as the flux decayed, and the non-thermal soft X-ray spectral component fading faster than the thermal one. Almost simultaneously to our second and fourth XMM–Newton observations (on 2008 August 29 and September 2), we observed the source in the hard X-ray range with INTEGRAL, which clearly detected the source up to ∼100 keV in the first pointing, while giving only upper limits during the second pointing, discovering a variable hard X-ray component fading in less than 10 days after the bursting activation. We performed a phase-coherent X-ray timing analysis over about 160 days starting with the burst activation and found evidence of a strong second derivative period component []. Thanks to the phase connection, we were able to study the phase-resolved spectral evolution of SGR 0501+4516 in great detail. We also report on the ROSAT quiescent source data, taken back in 1992 when the source exhibits a flux ∼80 times lower than that measured during the outburst, and a rather soft, thermal spectrum.
We present high-speed, multicolour optical photometry of the anomalous X-ray pulsar 4U 0142+61, obtained with ULTRACAM on
the 4.2-m William Herschel Telescope. We detect 4U 0142+61 at magnitudes of i′ = 23.7 ± 0.1, g′ = 27.2 ± 0.2 and u′ > 25.8, consistent with the magnitudes found recently by Hulleman et al. and hence confirming their discovery of both a
spectral break in the optical and a lack of long-term optical variability. We also confirm the earlier discovery of Kern &
Martin that 4U 0142+61 shows optical pulsations with an identical period (∼8.7 s) to the X-ray pulsations. The rms pulsed
fraction in our data is 29 ± 8 per cent, 5–7 times greater than the 0.2–8 keV X-ray rms pulsed fraction. The optical and X-ray
pulse profiles show similar morphologies and appear to be approximately in phase with each other, the former lagging the latter
by only 0.04 ± 0.02 cycles. In conjunction with the constraints imposed by X-ray observations, the results presented here
favour a magnetar interpretation for the anomalous X-ray pulsars.
ULTRACAM is a portable, high-speed imaging photometer designed to study faint astronomical objects at high temporal resolutions.
ULTRACAM employs two dichroic beamsplitters and three frame-transfer CCD cameras to provide three-colour optical imaging at
frame rates of up to 500 Hz. The instrument has been mounted on both the 4.2-m William Herschel Telescope on La Palma and
the 8.2-m Very Large Telescope in Chile, and has been used to study white dwarfs, brown dwarfs, pulsars, black hole/neutron
star X-ray binaries, gamma-ray bursts, cataclysmic variables, eclipsing binary stars, extrasolar planets, flare stars, ultracompact
binaries, active galactic nuclei, asteroseismology and occultations by Solar System objects (Titan, Pluto and Kuiper Belt
objects). In this paper we describe the scientific motivation behind ULTRACAM, present an outline of its design and report
on its measured performance.
SGR~0501+4516 was discovered with the Swift satellite on 2008 August 22, after it emitted a series of very energetic bursts. Since then, the source was extensively monitored with Swift, the Rossi X-ray Timing Explorer (RXTE) and observed with Chandra and XMM-Newton, providing a wealth of information about its outburst behavior and burst induced changes of its persistent X-ray emission. Here we report the most accurate location of SGR~0501+4516 (with an accuracy of 0.11'') derived with Chandra. Using the combined RXTE, Swift/X-ray Telescope, Chandra and XMM-Newton observations we construct a phase connected timing solution with the longest time baseline (~240 days) to date for the source. We find that the pulse profile of the source is energy dependent and exhibits remarkable variations associated with the SGR~0501+4516 bursting activity. We also find significant spectral evolution (hardening) of the source persistent emission associated with bursts. Finally, we discuss the consequences of the SGR~0501+4516 proximity to the supernova remnant, SNR G160.9+2.6 (HB9). Comment: Accepted for publication in the ApJ
Anomalous X-ray pulsars (AXPs) differ from ordinary radio pulsars in that their X-ray luminosity is orders of magnitude greater than their rate of rotational energy loss, and so they require an additional energy source. One possibility is that AXPs are highly magnetized neuron stars or 'magnetars' having surface magnetic fields greater than 10(14) G. This would make them similar to the soft gamma-ray repeaters (SGRs), but alternative models that do not require extreme magnetic fields also exist. An optical counterpart to the AXP 4U0142+61 was recently discovered, consistent with emission from a magnetar, but also from a magnetized hot white dwarf, or an accreting isolated neutron star. Here we report the detection of optical pulsations from 4U0142+61. The pulsed fraction of optical light (27 per cent) is five to ten times greater than that of soft X-rays, from which we conclude that 4U0142+61 is a magnetar. Although this establishes a direct relationship between AXPs and the soft gamma-ray repeaters, the evolutionary connection between AXPs, SGRs and radio pulsars remains controversial.
We examine the reprocessing of X-ray radiation from compact objects by accretion disks when the X-ray emission from the star is highly beamed. The reprocessed flux for various degrees of beaming and inclinations of the beam axis with respect to the disk is determined. We find that, in the case where the beam is produced by a non-relativistic object, the intensity of the emitted spectrum is highly suppressed if the beam is pointing away from the disk. However, for beams produced by compact objects, general relativistic effects cause only a small reduction in the reradiated flux even for very narrow beams oriented perpendicularly to the disk. This is especially relevant in constraining models for the anomalous X-ray pulsars, whose X-ray emission is highly beamed. We further discuss other factors that can influence the emission from disks around neutron stars. Comment: 11 pages with 4 figures, accepted to ApJ Letters
The similarity of rotation periods of, the anomalous X-ray pulsars (AXPs), the soft gamma ray repeaters (SGRs) and the dim thermal neutron stars (DTNs) suggests a common mechanism with an asymptotic spindown phase through the propeller and early accretion stages. The DTNs are in the propeller stage. Their luminosities arise from frictional heating in the neutron star. If the 8.4 s rotation period of the DTN RXJ 0720.4-3125 is close to its rotational equilibrium period, the propeller torque indicates a magnetic field in the 10$^{12}$ Gauss range. The mass inflow rate onto the propeller is of the order of the AXP accretion rates. The limited range of rotation periods, taken to be close to equilibrium periods, and magnetic fields in the range 5 E11- 5 E12 Gauss correspond to mass inflow rates 3.2 E14 gm/s < \dot{M} < 4.2 E17 gm/s. Observed spindown rates of the AXPs and SGRs also fit in with these fields rather than magnetar fields periods. The source of the mass inflow is a remnant accretion disk formed as part of the fallback during the supernova explosion. These classes of sources thus represent the alternative pathways for those neutron stars that do not become radio pulsars. For the highest mass inflow rates the propeller action may support enough circumstellar material so that the optical thickness to electron scattering destroys the X-ray beaming, and the rotation period is not observable. These are the radio quiet neutron stars (RQNSs) at the centers of supernova remnants Cas A, Puppis A, RCW 103 and 296.5+10. Comment: 28 pages, with one figure and one table. Submitted to ApJ
Two classes of X-ray pulsars, the Anomalous X-ray Pulsars and the Soft Gamma-ray Repeaters, have been recognized in the last decade as the most promising candidates for being magnetars: isolated neutron stars powered by magnetic energy. I review the observational properties of these objects, focussing on the most recent results, and their interpretation in the magnetar model. Alternative explanations, in particular those based on accretion from residual disks, are also considered. The possible relations between these sources and other classes of neutron stars and astrophysical objects are also discussed. Comment: Submitted to Astromomy and Astrophysics Review (57 pages, 19 figures, 6 tables)
This article is a review of Soft Gamma Repeaters and Anomalous X-ray Pulsars. It contains a brief historical record of the emergence of these classes of neutron stars, a thorough overview of the observational data, a succinct summary of the magnetar model, and suggested directions for future research in this field.
We show that the observed pulsed optical emission of the anomalous X-ray pulsar 4U 0142+61 can be accounted for by both the magnetar outer gap models and the disk-star dynamo gap models, therefore is not an evidence in favor of one of these models as its responsible mechanism. Nevertheless, the estimated high energy gamma-ray spectra from these models have different power-low indices, and can be tested by future observations of the Gamma-ray Large-Area Space Telescope (GLAST). Furthermore, we show by analytical estimations that the expectations of a standard disk model is in agreement with the observed unpulsed optical and infrared luminosities of the AXP 4U 0142+61. Comment: 14 pages, 1 figure, accepted for publication in ApJ
We present a model for the anomalous X-ray pulsars (AXPs) in which the emission is powered by accretion from a fossil disk, established from matter falling back onto the neutron star following its birth. The time-dependent accretion drives the neutron star towards a ``tracking'' solution in which the rotation period of the star increases slowly, in tandem with the declining accretion rate. For appropriate choices of disk mass, neutron star magnetic field strength and initial spin period, we demonstrate that a rapidly rotating neutron star can be spun down to periods characteristic of AXPs on timescales comparable to the estimated ages of these sources. In other cases, accretion onto the neutron star switches off after a short time, and the star becomes an ordinary radio pulsar. Thus, in our picture, radio pulsars and AXPs are drawn from the same underlying population, in contrast to models involving neutron stars with ultrastrong magnetic fields, which require a new population of stars with very different properties. Comment: 15 pages and 3 Postscript figures
We present K'-band (2.12 micron) imaging observations of the SGR 1806-20 field taken during its very active phase in mid 2004, which reveal brightening of sources within the Chandra X-ray error circle when compared with earlier images obtained in 2002. One source brightened by more than a factor of 2, and so we consider this to be the probable infrared counterpart for SGR 1806-20. The other two sources are located in close proximity to the probable counterpart and show marginal brightening, which may suggest that the high-energy photons emitted from the SGR during its active phase have induced dust sublimation or brightening of the unresolved background around the SGR. Comment: Accepted for publication in ApJ Letters. (12 pages, 3 figures)
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