arXiv subjects

A. P. Beardmore

Publications and source records attributed to A. P. Beardmore.

At least 109 records · Page 6Linked to original sources

arXiv2006

A refined position catalog of the Swift XRT afterglows

We present a catalogue of refined positions of 68 gamma ray burst (GRB) afterglows observed by the Swift X-ray Telescope (XRT) from the launch up to 2005 Oct 16. This is a result of the refinement of the XRT boresight calibration. We tested this correction by means of a systematic study of a large sample of X-ray sources observed by XRT with well established optical counterparts. We found that we can reduce the systematic error radius of the measurements by a factor of two, from 6.5" to 3.2" (90% of confidence). We corrected all the positions of the afterglows observed by XRT in the first 11 months of the Swift mission. This is particularly important for the 37 X-ray afterglows without optical counterpart. Optical follow-up of dark GRBs, in fact, will be more efficient with the use of the more accurate XRT positions.

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arXiv2006

Evidence for a Canonical GRB Afterglow Light Curve in the Swift/XRT Data

We present new observations of the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) detected with the Swift X-ray Telescope (XRT). The early X-ray afterglows show a canonical behavior, where the light curve broadly consists of three distinct power law segments: (i) an initial very steep decay (t^{-alpha} with 3 4, and augments the already severe requirements for the efficiency of the prompt gamma-ray emission.

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arXiv2006

X-ray flare in XRF 050406: evidence for prolonged engine activity

We present observations of XRF 050406, the first burst detected by Swift showing a flare in its X-ray light curve. During this flare, which peaks at t_peak ~210s after the BAT trigger, a flux variation of (delta F)/F~6 in a very short time (delta t)/t_peak<<1 was observed. Its measured fluence in the 0.2-10 keV band was ~1.4x10^-8 erg cm^-2, which corresponds to 1-15% of the prompt fluence. We present indications of spectral variations during the flare. We argue that the producing mechanism is late internal shocks, which implies that the central engine is still active at 210s, though with a reduced power with respect to the prompt emission. The X-ray light curve flattens to a very shallow slope with decay index of ~0.5 after ~4400s, which also supports continued central engine activity at late times. This burst is classified as an X-ray flash, with a relatively low fluence (~10^-7 erg cm^-2 in the 15-350 keV band, E_iso~10^51 erg), a soft spectrum (photon index 2.65), no significant flux above ~50 keV and a peak energy E_p<15 keV. XRF 050406 is one of the first examples of a well-studied X-ray light curve of an XRF. We show that the main afterglow characteristics are qualitatively similar to those of normal GRBs. In particular, X-ray flares superimposed on a power-law light curve have now been seen in both XRFs and GRBs. This indicates that a similar mechanism may be at work for both kinds of events.

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arXiv2005

The Giant X-ray Flare of GRB 050502B: Evidence for Late-Time Internal Engine Activity

Until recently, X-ray flares during the afterglow of gamma ray bursts (GRBs) were a rarely detected phenomenon, thus their nature is unclear. During the afterglow of GRB 050502B, the largest X-ray flare ever recorded rose rapidly above the afterglow lightcurve detected by the Swift X-ray Telescope. The peak flux of the flare was >500 times that of the underlying afterglow, and it occurred at >12 minutes after the nominal prompt burst emission. The fluence of this X-ray flare, (1.0 +/- 0.05) x 10^{-6} erg cm^{-2} in the 0.2-10.0 keV energy band, exceeded the fluence of the nominal prompt burst. The spectra during the flare were significantly harder than those measured before and after the flare. Later in time, there were additional flux increases detected above the underlying afterglow, as well as a break in the afterglow lightcurve. All evidence presented below, including spectral and particularly timing information during and around the giant flare, suggests that this giant flare was the result of internal dissipation of energy due to late central engine activity, rather than an afterglow-related effect. We also find that the data are consistent with a second central engine activity episode, in which the ejecta is moving slower than that of the initial episode, causing the giant flare and then proceeding to overtake and refresh the afterglow shock, thus causing additional activity at even later times in the lightcurve.

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arXiv2005

GRB 050911: a black hole - neutron star merger or a naked GRB

GRB 050911, discovered by the Swift Burst Alert Telescope, was not seen 4.6 hr later by the Swift X-ray Telescope, making it one of the very few X-ray non-detections of a Gamma-Ray Burst (GRB) afterglow at early times. The gamma-ray light-curve shows at least three peaks, the first two of which (~T_0 - 0.8 and T_0 + 0.2 s, where T_0 is the trigger time) were short, each lasting 0.5 s. This was followed by later emission 10-20 s post-burst. The upper limit on the unabsorbed X-ray flux was 1.7 x 10^-14 erg cm^-2 s^-1 (integrating 46 ks of data taken between 11 and 18 September), indicating that the decay must have been rapid. All but one of the long bursts detected by Swift were above this limit at ~4.6 hr, whereas the afterglows of short bursts became undetectable more rapidly. Deep observations with Gemini also revealed no optical afterglow 12 hr after the burst, down to r=24.0 (5-sigma limit). We speculate that GRB 050911 may have been formed through a compact object (black hole-neutron star) merger, with the later outbursts due to a longer disc lifetime linked to a large mass ratio between the merging objects. Alternatively, the burst may have occured in a low density environment, leading to a weak, or non-existent, forward shock - the so-called 'naked GRB' model.

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arXiv2005

Evidence for intrinsic absorption in the Swift X-ray afterglows

Gamma-ray burst (GRB) progenitors are observationally linked to the death of massive stars. X-ray studies of the GRB afterglows can deepen our knowledge of the ionization status and metal abundances of the matter in the GRB environment. Moreover, the presence of local matter can be inferred through its fingerprints in the X-ray spectrum, i.e. the presence of absorption higher than the Galactic value. A few studies based on BeppoSAX and XMM-Newton found evidence of higher than Galactic values for the column density in a number of GRB afterglows. Here we report on a systematic analysis of 17 GRBs observed by Swift up to April 15, 2005. We observed a large number of GRBs with an excess of column density. Our sample, together with previous determinations of the intrinsic column densities for GRBs with known redshift, provides evidence for a distribution of absorption consistent with that predicted for randomly occurring GRB within molecular clouds.

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arXiv2005

The dust-scattered X-ray halo around Swift GRB 050724

This paper discusses the X-ray halo around the Swift gamma-ray burst GRB 050724 (z=0.258), detected by the Swift X-Ray Telescope. The halo, which forms a ring around the fading X-ray source, expands to a radius of 200" within 8 ks of the burst exactly as expected for small-angle X-ray scattering by Galactic dust along the line of sight to a cosmologically distant GRB. The expansion curve and radial profile of the halo constrain the scattering dust to be concentrated at a distance of D = 139 +/- 9 pc (from Earth) in a cloud/sheet of thickness delta-D < 22 pc. The halo was observed only out to scattering angles of 200", for which the scattering is dominated by the largest grains, with a maximum size estimated to be a_max ~ 0.4-0.5 um. The scattering-to-extinction ratio was estimated to be tau_scat/A_V > 0.022; this is a lower limit to the true value because contribution from smaller grains, which scatter to larger angles, was not directly observed. The line-of-sight to the GRB passes close to the Ophiuchus molecular cloud complex, which provides a plausible site for the scattering dust.

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arXiv2005

Swift observations of the X-ray bright GRB 050315

This paper discusses Swift observations of the gamma-ray burst GRB 050315 (z=1.949) from 80 s to 10 days after the onset of the burst. The X-ray light curve displayed a steep early decay (t^-5) for ~200 s and several breaks. However, both the prompt hard X-ray/gamma-ray emission (observed by the BAT) and the first ~ 300 s of X-ray emission (observed by the XRT) can be explained by exponential decays, with similar decay constants. Extrapolating the BAT light curve into the XRT band suggests the rapidly decaying, early X-ray emission was simply a continuation of the fading prompt emission; this strong similarity between the prompt gamma-ray and early X-ray emission may be related to the simple temporal and spectral character of this X-ray rich GRB. The prompt (BAT) spectrum was a steep down to 15 keV, and appeared to continue through the XRT bandpass, implying a low peak energy, inconsistent with the Amati relation. Following the initial steep decline the X-ray afterglow did not fade for ~1.2*10^4 s, after which time it decayed with a temporal index of alpha ~ 0.7, followed by a second break at ~2.5*10^5 s to a slope of alpha ~ 2. The apparent `plateau' in the X-ray light curve, after the early rapid decay, makes this one of the most extreme examples of the steep-flat-steep X-ray light curves revealed by Swift. If the second afterglow break is identified with a jet break then the jet opening angle was theta_0 ~ 5 deg, and implying E_gamma > 10^50 erg.

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arXiv2005

The in-flight spectroscopic performance of the Swift XRT CCD camera

The Swift X-ray Telescope (XRT) focal plane camera is a front-illuminated MOS CCD, providing a spectral response kernel of 144 eV FWHM at 6.5 keV. We describe the CCD calibration program based on celestial and on-board calibration sources, relevant in-flight experiences, and developments in the CCD response model. We illustrate how the revised response model describes the calibration sources well. Loss of temperature control motivated a laboratory program to re-optimize the CCD substrate voltage, we describe the small changes in the CCD response that would result from use of a substrate voltage of 6V.

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arXiv2005

GRB 050117: Simultaneous Gamma-ray and X-ray Observations with the Swift Satellite

The Swift Gamma-Ray Burst Explorer performed its first autonomous, X-ray follow-up to a newly detected GRB on 2005 January 17, within 193 seconds of the burst trigger by the Swift Burst Alert Telescope. While the burst was still in progress, the X-ray Telescope obtained a position and an image for an un-catalogued X-ray source; simultaneous with the gamma-ray observation. The XRT observed flux during the prompt emission was 1.1 x 10^{-8} ergs cm^{-2} s^{-1} in the 0.5-10 keV energy band. The emission in the X-ray band decreased by three orders of magnitude within 700 seconds, following the prompt emission. This is found to be consistent with the gamma-ray decay when extrapolated into the XRT energy band. During the following 6.3 hours, the XRT observed the afterglow in an automated sequence for an additional 947 seconds, until the burst became fully obscured by the Earth limb. A faint, extremely slowly decaying afterglow, alpha=-0.21$, was detected. Finally, a break in the lightcurve occurred and the flux decayed with alpha<-1.2$. The X-ray position triggered many follow-up observations: no optical afterglow could be confirmed, although a candidate was identified 3 arcsecs from the XRT position.

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arXiv2005

Swift XRT Observations of the Afterglow of GRB 050319

Swift discovered the high redshift GRB 050319 with the Burst Alert Telescope and began observing with its narrow field instruments only 225 s after the burst onset. The afterglow X-ray emission was monitored by the XRT up to 28 days after the burst. The light curve shows a decay with three different phases, each characterized by a distinct slope: an initial steep decay with a power law index of ~ 5.5, a second phase characterized by a flat decay slope of \~ 0.54, and a third phase with a decay slope of ~ 1.14. During the first phase the spectral energy distribution is softer than in the following two phases and the photon index is consistent with the GRB prompt spectrum. The extrapolation of the BAT light curve to the XRT band suggests that the initial fast decaying phase of the XRT afterglow might be the low energy tail of the prompt emission. The second break in the afterglow light curve occurs about 27000 s after the burst. The spectral energy distribution before and after the second break does not change and it can be tentatively interpreted as a jet break or the end of a delayed or continuous energy injection phase.

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arXiv2005

Swift XRT Observations of the Breaking X-ray Afterglow of GRB 050318

We report the results of Swift X-Ray Telescope (XRT) observations of GRB 050318. This event triggered the Burst Alert Telescope (BAT) aboard Swift and was followed-up with XRT and UVOT for 11 consecutive orbits starting from 54 minutes after the trigger. A previously unknown fading X-ray source was detected and accurately monitored. The source was found to decrease in intensity with time and a clear temporal break occurring at ~18000 s after the trigger was observed. The X-ray light curve was found to be consistent with a broken power-law with decay indices -1.17 +/- 0.08 and -2.10 (+0.22) (-0.24) before and after the break. The spectrum of the X-ray afterglow was well described by a photoelectrically absorbed power-law with energy index of -1.09 +/-0.09. No evidence of spectral evolution was found. We compare these results with those obtained with UVOT and separately reported and refine the data analysis of BAT. We discuss our results in the framework of a collimated fireball model and a synchrotron radiation emission mechanism. Assuming the GRB redshift derived from the farthest optical absorption complex (z = 1.44), the event is fully consistent with the E_p-E_iso correlation.

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arXiv2005

Prompt and afterglow early X-ray phases in the comoving frame. Evidence for Universal properties?

We analyze the Swift XRT light curves and spectra of the gamma-ray bursts (GRBs) for which the redshift has been measured. The sample consists of seven GRBs. The soft X-ray light curves of all these GRBs are separated into at least two morphological classes: 1) those starting off with a very steep light curve decay and 2) those showing a rather mild initial decay. This initial decay is followed by a flattening and by a further steepening. During these transitions the soft X-ray spectrum of these GRBs remains constant within the observational errors (except for one case, GRB050319). For the first time we are able to exploit the early light curve of GRB afterglows in the comoving frame. Besides the temporal and spectral behavior we find that the energy of the afterglow emitted in the (rest frame) time interval 20-200 s and 1300-12600 s after the trigger correlates with the mean energy of the prompt emission, hinting at a close link between the two. Averaging over the bursts that have a rather well sampled light curve and starting immediately after the prompt phase, the energy emitted in the soft X-rays is about 3% of the energy emitted during the prompt phase, except for GRB050315, where the soft X-ray emission is as high as 14% of the prompt emission.

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arXiv2005

Bright X-ray Flares in Gamma-Ray Burst Afterglows

Gamma-ray burst (GRB) afterglows have provided important clues to the nature of these massive explosive events, providing direct information on the nearby environment and indirect information on the central engine that powers the burst. We report the discovery of two bright X-ray flares in GRB afterglows, including a giant flare comparable in total energy to the burst itself, each peaking minutes after the burst. These strong, rapid X-ray flares imply that the central engines of the bursts have long periods of activity, with strong internal shocks continuing for hundreds of seconds after the gamma-ray emission has ended.

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arXiv2005

GRB050223: A faint Gamma-Ray Burst discovered by Swift

GRB050223 was discovered by the Swift Gamma-Ray Burst Explorer on 23 February 2005 and was the first Gamma-Ray Burst to be observed by both Swift and XMM-Newton. At the time of writing (May 2005), it has one of the faintest GRB afterglows ever observed. The spacecraft could not slew immediately to the burst, so the first X-ray and optical observations occurred approximately 45 minutes after the trigger. Although no optical emission was found by any instrument, both Swift and XMM-Newton detected the fading X-ray afterglow. Combined data from both of these observatories show the afterglow to be fading monotonically as 0.99 +0.15/-0.12 over a time frame between 45 minutes to 27 hours post-burst. Spectral analysis, allowed largely by the higher through-put of XMM-Newton, implies a power-law with a slope of Gamma=1.75 +0.19/-0.18 and shows no evidence for absorption above the Galactic column of 7 x 10^20 cm^-2. From the X-ray decay and spectral slopes, a low electron power-law index of p = 1.3-1.9 is derived; the slopes also imply that a jet-break has not occured up to 27 hours after the burst. The faintness of GRB050223 may be due to a large jet opening or viewing angle or a high redshift.

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arXiv2005

An unexpectedly rapid decline in the X-ray afterglow emission of long gamma-ray bursts

Long gamma-ray bursts (GRBs) are commonly accepted to originate in the explosion of particularly massive stars, which gives rise to a highly relativistic jet. Internal inhomogeneities in the expanding flow give rise to internal shock waves that are believed to produce the gamma-rays we see. As the jet travels further outward into the surrounding circumstellar medium `external' shocks give rise to the afterglow emission seen in the X-ray, optical and radio bands. Here we report on the early phases of the X-ray emission of five GRBs. Their X-ray light curves are characterised by a rapid fall-off for the first few hundred seconds, followed by a less rapid decline lasting several hours. This steep decline, together with detailed spectral properties of two particular bursts, shows that violent shock interactions take place in the early jet outflows.

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arXiv2005

Ultraviolet, Optical, and X-Ray Observations of the Type Ia Supernova 2005am with Swift

We present ultraviolet and optical light curves in six broadband filters and grism spectra obtained by Swift's Ultraviolet/Optical Telescope for the Type Ia supernova SN2005am. The data were collected beginning about four days before the B-band maximum, with excellent coverage of the rapid decline phase and later observations extending out to 69 days after the peak. The optical and near UV light curve match well those of SN1992A. The other UV observations constitute the first set of light curves shorter than 2500 Angstroms and allow us to compare the light curve evolution in three UV bands. The UV behavior of this and other low redshift supernovae can be used to constrain theories of progenitor evolution or to interpret optical light curves of high redshift supernovae. Using Swift's X-Ray Telescope, we also report the upper limit to SN2005am's X-ray luminosity to be 1.77 x 10^40 erg s^-1 in the 0.3--10 keV range from 58,117 s of exposure time.

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arXiv2005

Swift observations of GRB050128: the early X-ray afterglow

Swift discovered GRB050128 with the Burst Alert Telescope and promptly pointed its narrow field instruments to monitor the afterglow. X-ray observations started 108 s after the trigger time. The early decay of the afterglow is relatively flat with a temporal decay modeled with a power law with index ~ -0.3. A steepening occurs at later times (~ 1500 s) with a power law index of ~ -1.3. During this transition, the observed X-ray spectrum does not change. We interpret this behaviour as either an early jet break or evidence for a transition from the fast cooling regime to the slow cooling regime in a wind environment.

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