Olivier Godet

Methods and results of an automatic analysis of a complete sample of Swift-XRT observations of GRBs

We present a homogeneous X-ray analysis of all 318 gamma-ray bursts detected by the X-ray telescope (XRT) on the Swift satellite up to 2008 July 23; this represents the largest sample of X-ray GRB data published to date. In Sections 2-3, we detail the methods which the Swift-XRT team has developed to produce the enhanced positions, light curves, hardness ratios and spectra presented in this paper. Software using these methods continues to create such products for all new GRBs observed by the Swift-XRT. We also detail web-based tools allowing users to create these products for any object observed by the XRT, not just GRBs. In Sections 4-6, we present the results of our analysis of GRBs, including probability distribution functions of the temporal and spectral properties of the sample. We demonstrate evidence for a consistent underlying behaviour which can produce a range of light-curve morphologies, and attempt to interpret this behaviour in the framework of external forward shock emission. We find several difficulties, in particular that reconciliation of our data with the forward shock model requires energy injection to continue for days to weeks.

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. These power law segments are separated by two corresponding break times. On top of this canonical behavior of the early X-ray light curve, many events have superimposed X-ray flares, which are most likely caused by internal shocks due to long lasting sporadx activity of the central engine, up to several hours after the GRB. We find that the initial steep decay is consistent with it being the tail of the prompt emission: from photons that are radiated at large angles relative to our line of sight. The first break in the light curve takes place when the forward shock emission becomes dominant, with the intermediate shallow flux decay likely caused by the continuous energy injection into the external shock. When this energy injection stops, a second break is then observed in the light curve. This energy injection increases the energy of the afterglow shock by at least a factor of f greater than or approx. equal to 4, and augments the already severe requirements for the efficiency of the prompt gamma-ray emission.

Evidence for a canonical gamma-ray burst 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) well observed by 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: (1) an initial very steep decay (/t � � with 3P � 1 P5), followed by (2) a very shallow decay (0:5P � 2 P1:0), and finally (3) a somewhat steeper decay (1P � 3 P1:5). These power-law segments are separated by two corresponding break times, tbreak;1 P500 s and 10 3 sPtbreak;2P 10 4 s. On top of this canonical behavior, many events have superimposed X-ray flares, which are most likely caused by internal shocks due to long-lasting sporadic activity of the central engine, up to several hours after the GRB. We find that the initial steep decay is consistent with it being the tail of the prompt emission, from photons that are radiated at large angles relative to our line of sight. The first break in the light curve (tbreak;1) takes place when the forward shock emission becomes dominant, with the intermediate shallow flux decay (� 2) likely caused by the continuous energy injection into the external shock. When this energy injection stops, a second break is then observed in the light curve (tbreak;2). This energy injection increases the energy of the afterglow shock by at least a factor of f k4 and augments the already severe requirements for the efficiency of the prompt gamma-ray emission. Subject headingg gamma rays: bursts — radiation mechanisms: nonthermal

The association of GRB 060218 with a supernova and the evolution of the shock wave.

Although the link between long Gamma Ray Bursts (GRBs) and supernovae (SNe) has been established, hitherto there have been no observations of the beginning of a supernova explosion and its intimate link to a GRB. In particular, we do not know however how a GRB jet emerges from the star surface nor how a GRB progenitor explodes. Here we report on observations of the close GRB060218 and its connection to SN2006aj. In addition to the classical non-thermal emission, GRB060218 shows a thermal component in its X-ray spectrum, which cools and shifts into the optical/UV band as time passes. We interpret these features as arising from the break out of a shock driven by a mildly relativistic shell into the dense wind surrounding the progenitor. Our observations allow us for the first time to catch a SN in the act of exploding, to directly observe the shock break-out and to provide strong evidence that the GRB progenitor was a Wolf-Rayet star.Although the link between long γ-ray bursts (GRBs) and supernovae has been established, hitherto there have been no observations of the beginning of a supernova explosion and its intimate link to a GRB. In particular, we do not know how the jet that defines a γ-ray burst emerges from the stars surface, nor how a GRB progenitor explodes. Here we report observations of the relatively nearby GRB 060218 (ref. 5) and its connection to supernova SN 2006aj (ref. 6). In addition to the classical non-thermal emission, GRB 060218 shows a thermal component in its X-ray spectrum, which cools and shifts into the optical/ultraviolet band as time passes. We interpret these features as arising from the break-out of a shock wave driven by a mildly relativistic shell into the dense wind surrounding the progenitor. We have caught a supernova in the act of exploding, directly observing the shock break-out, which indicates that the GRB progenitor was a Wolf–Rayet star.

The early x-ray emission from grbs

We present observations of the early X-ray emission for a sample of 40 gamma-ray bursts (GRBs) obtained using the Swift satellite, for which the narrow-field instruments were pointed at the burst within 10 minutes of the trigger. Using data from the Burst Alert Telescope and the X-Ray Telescope, we show that the X-ray light curve can be well described by an exponential that relaxes into a power law, often with flares superimposed. The transition time between the exponential and the power law provides a physically defined timescale for the burst duration. In most bursts, the power law breaks to a shallower decay within the first hour, and a late emission hump is observed, which can last for many hours. In other GRBs the hump is weak or absent. The observed variety in the shape of the early X-ray light curve can be explained as a combination of three components: prompt emission from the central engine, afterglow, and the late hump. In this scenario, afterglow emission begins during or soon after the burst, and the observed shape of the X-ray light curve depends on the relative strengths of the emission due to the central engine and that of the afterglow. There is a strong correlation such that those GRBs with stronger afterglow components have brighter early optical emission. The late emission hump can have a total fluence equivalent to that of the prompt phase. GRBs with the strongest late humps have weak or no X-ray flares.

An intermediate-mass black hole of over 500 solar masses in the galaxy ESO 243-49

Ultraluminous X-ray sources are extragalactic objects located outside the nucleus of the host galaxy with bolometric luminosities exceeding 1039 erg s-1. These extreme luminosities—if the emission is isotropic and below the theoretical (Eddington) limit, where the radiation pressure is balanced by the gravitational pressure—imply the presence of an accreting black hole with a mass of ∼102–105 solar masses (). The existence of such intermediate-mass black holes is in dispute, and though many candidates have been proposed, none are widely accepted as definitive. Here we report the detection of a variable X-ray source with a maximum 0.2–10 keV luminosity of up to 1.1 × 1042 erg s-1 in the edge-on spiral galaxy ESO 243-49, with an implied conservative lower limit for the mass of the black hole of ∼500.

Testing the Standard Fireball Model of Gamma-Ray Bursts Using Late X-Ray Afterglows Measured by Swift

We show that all X-ray decay curves of γ-ray bursts (GRBs) measured by Swift can be fitted using one or two components, both of which have exactly the same functional form comprised of an early falling exponential phase followed by a power-law decay. The first component contains the prompt γ-ray emission and the initial X-ray decay. The second component appears later, has a much longer duration, and is present for ≈80% of GRBs. It most likely arises from the external shock that eventually develops into the X-ray afterglow. In the remaining ≈20% of GRBs the initial X-ray decay of the first component fades more slowly than the second and dominates at late times to form an afterglow. The temporal decay parameters and γ/X-ray spectral indices derived for 107 GRBs are compared to the expectations of the standard fireball model including a search for possible jet breaks. For ~50% of GRBs the observed afterglow is in accord with the model, but for the rest the temporal and spectral indices do not conform to the expected closure relations and are suggestive of continued, late, energy injection. We identify a few possible jet breaks, but there are many examples where such breaks are predicted but are absent. The time Ta at which the exponential phase of the second component changes to a final power-law decay afterglow is correlated with the peak of the γ-ray spectrum, Epeak. This is analogous to the Ghirlanda relation, indicating that this time is in some way related to optically observed break times measured for pre-Swift bursts.

The 22-Month Swift-BAT All-Sky Hard X-ray Survey

We present the catalog of sources detected in the first 22 months of data from the hard X-ray survey (14-195 keV) conducted with the Burst Alert Telescope (BAT) coded mask imager on the Swift satellite. The catalog contains 461 sources detected above the 4.8σ level with BAT. High angular resolution X-ray data for every source from Swift-XRT or archival data have allowed associations to be made with known counterparts in other wavelength bands for over 97% of the detections, including the discovery of ~30 galaxies previously unknown as active galactic nuclei and several new Galactic sources. A total of 266 of the sources are associated with Seyfert galaxies (median redshift z ~ 0.03) or blazars, with the majority of the remaining sources associated with X-ray binaries in our Galaxy. This ongoing survey is the first uniform all-sky hard X-ray survey since HEAO-1 in 1977. Since the publication of the nine-month BAT survey we have increased the number of energy channels from four to eight and have substantially increased the number of sources with accurate average spectra. The BAT 22 month catalog is the product of the most sensitive all-sky survey in the hard X-ray band, with a detection sensitivity (4.8σ) of 2.2 × 10–11 erg cm–2 s–1 (1 mCrab) over most of the sky in the 14-195 keV band.

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

‘Long’ γ-ray bursts (GRBs) are commonly accepted to originate in the explosion of particularly massive stars, which give rise to highly relativistic jets. Inhomogeneities in the expanding flow result in internal shock waves that are believed to produce the γ-rays we see. As the jet travels further outward into the surrounding circumstellar medium, ‘external’ shocks create the afterglow emission seen in the X-ray, optical and radio bands. Here we report observations of the early phases of the X-ray emission of five GRBs. Their X-ray light curves are characterised by a surprisingly 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.

SWIFT OBSERVATIONS OF THE X-RAY-BRIGHT GRB 050315

This paper discusses Swift observations of the � -ray burst GRB 050315 (z ¼ 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/� -ray emission (observed by the BAT) and the first � 300 s of X-ray emission (observed bytheXRT)canbeexplainedbyexponentialdecays,withsimilardecayconstants.ExtrapolatingtheBATlightcurve into the XRT band suggests that the rapidly decaying, early X-ray emission was simply a continuation of the fading promptemission;thisstrongsimilaritybetweentheprompt � -rayandearlyX-rayemissionmayberelatedtothesimple temporal and spectral character of this X-ray–rich GRB. Theprompt (BAT) spectrum was steep down to � 15keVand 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 at emporal index of� � 0:7, followed by a second break at � 2:5 ; 10 5 s to a slope of � � 2. The apparent ‘‘plateau’’ in the X-raylight 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