D. N. Burrows

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 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

Relativistic jet activity from the tidal disruption of a star by a massive black hole [Discovery of the onset of rapid accretion by a dormant massive black hole]

Supermassive black holes have powerful gravitational fields with strong gradients that can destroy stars that get too close, producing a bright flare in ultraviolet and X-ray spectral regions from stellar debris that forms an accretion disk around the black hole. The aftermath of this process may have been seen several times over the past two decades in the form of sparsely sampled, slowly fading emission from distant galaxies, but the onset of the stellar disruption event has not hitherto been observed. Here we report observations of a bright X-ray flare from the extragalactic transient Swift J164449.3+573451. This source increased in brightness in the X-ray band by a factor of at least 10,000 since 1990 and by a factor of at least 100 since early 2010. We conclude that we have captured the onset of relativistic jet activity from a supermassive black hole. A companion paper comes to similar conclusions on the basis of radio observations. This event is probably due to the tidal disruption of a star falling into a supermassive black hole, but the detailed behaviour differs from current theoretical models of such events.D. N. Burrows , J. A. Kennea , G. Ghisellini , V. Mangano , B. Zhang , K. L. Page , M. Eracleous , P. Romano , T. Sakamoto , A. D. Falcone , J. P. Osborne , S. Campana , A. P. Beardmore , A. A. Breeveld , M. M. Chester , R. Corbet , S. Covino , J. R. Cummings , P. D’Avanzo , V. D’Elia , P. Esposito , P. A. Evans , D. Fugazza, J. M. Gelbord , K. Hiroi , S. T. Holland , K. Y. Huang , M. Im, G. Israel , Y. Jeon , Y.-B. Jeon , N. Kawai , H. A. Krimm , P. Mészáros , H. Negoro , N. Omodei , W.K. Park , J. S. Perkins , M. Sugizaki , H.-I. Sung , G. Tagliaferri , E. Troja , Y. Ueda, Y. Urata, R. Usui , L. A. Antonelli , S. D. Barthelmy , G. Cusumano , P. Giommi , F. E. Marshall , A. Melandri , M. Perri , J. L. Racusin , B. Sbarufatti , M. H. Siegel , & N. Gehrels 21

Analysis prcedures for ROSAT XRT/PSPC observations of extended objects and the diffuse background

Proper reduction of ROSAT observations of extended sources or the diffuse background requires techniques that are quite different from the standard point-source analysis. We describe in detail an appropriate set of procedures for this purpose for the X-ray Telescope (XRT), Position Sensitive Proportional Counter (PSPC). We define a standard set of pulse-height bands recommended for analyzing extended structure and describe the use of detector efficiency maps for proper flat-fielding of images. Sources of noncosmic background contamination and their modeling and subtraction are discussed and demonstrated. IDL routines for implementing these data reduction procedures are being made available to the community through the ROSAT IDL library maintained at Goddard Space Flight Center (GSFC).

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.

A γ-ray burst at a redshift of z ≈ 8.2

Long-duration gamma-ray bursts (GRBs) are thought to result from the explosions of certain massive stars, and some are bright enough that they should be observable out to redshifts of z > 20 using current technology. Hitherto, the highest redshift measured for any object was z = 6.96, for a Lyman-alpha emitting galaxy. Here we report that GRB 090423 lies at a redshift of z approximately 8.2, implying that massive stars were being produced and dying as GRBs approximately 630 Myr after the Big Bang. The burst also pinpoints the location of its host galaxy.It is thought that the first generations of massive stars in the Universe were an important, and quite possibly dominant, source of the ultra-violet radiation that reionized the hydrogen gas in the intergalactic medium (IGM); a state in which it has remained to the present day. Measurements of cosmic microwave background anisotropies suggest that this phase-change largely took place in the redshift range z=10.8 +/- 1.4, while observations of quasars and Lyman-alpha galaxies have shown that the process was essentially completed by z=6. However, the detailed history of reionization, and characteristics of the stars and proto-galaxies that drove it, remain unknown. Further progress in understanding requires direct observations of the sources of ultra-violet radiation in the era of reionization, and mapping the evolution of the neutral hydrogen fraction through time. The detection of galaxies at such redshifts is highly challenging, due to their intrinsic faintness and high luminosity distance, whilst bright quasars appear to be rare beyond z~7. Here we report the discovery of a gamma-ray burst, GRB 090423, at redshift z=8.26 -0.08 +0.07. This is well beyond the redshift of the most distant spectroscopically confirmed galaxy (z=6.96) and quasar (z=6.43). It establishes that massive stars were being produced, and dying as GRBs, ~625 million years after the Big Bang. In addition, the accurate position of the burst pinpoints the location of the most distant galaxy known to date. Larger samples of GRBs beyond z~7 will constrain the evolving rate of star formation in the early universe, while rapid spectroscopy of their afterglows will allow direct exploration of the progress of reionization with cosmic time.Long-duration γ-ray bursts (GRBs) are thought to result from the explosions of certain massive stars, and some are bright enough that they should be observable out to redshifts of z > 20 using current technology. Hitherto, the highest redshift measured for any object was z = 6.96, for a Lyman-α emitting galaxy. Here we report that GRB 090423 lies at a redshift of z ≈ 8.2, implying that massive stars were being produced and dying as GRBs ∼630 Myr after the Big Bang. The burst also pinpoints the location of its host galaxy.

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.

The First Survey of X-Ray Flares from Gamma-Ray Bursts Observed by Swift: Temporal Properties and Morphology

We present the first systematic investigation of the morphological and timing properties of flares in GRBs observed by Swift XRT. We consider a large sample drawn from all GRBs detected by Swift, INTEGRAL, and HETE-2 prior to 2006 January 31, which had an XRT follow-up and which showed significant flaring. Our sample of 33 GRBs includes long and short, at low and high redshift, and a total of 69 flares. The strongest flares occur in the early phases, with a clear anticorrelation between the flare peak intensity and the flare time of occurrence. Fitting each X-ray flare with a Gaussian model, we find that the mean ratio of the width and peak time is --> ? t/t = 0.13 ? 0.10, albeit with a large scatter. Late flares at times >2000 s have long durations, -->? t > 300 s, and can be very energetic compared to the underlying continuum. We further investigated whether there is a clear link between the number of pulses detected in the prompt phase by BAT and the number of X-ray flares detected by XRT, finding no correlation. However, we find that the distribution of intensity ratios between successive BAT prompt pulses and that between successive XRT flares is the same, an indication of a common origin for gamma-ray pulses and X-ray flares. All evidence indicates that flares are indeed related to the workings of the central engine and, in the standard fireball scenario, originate from internal shocks rather than external shocks. While all flares can be explained by long-lasting engine activity, 29/69 flares may also be explained by refreshed shocks. However, 10 can only be explained by prolonged activity of the central engine.

Broadband observations of the naked-eye gamma-ray burst GRB 080319B

Long-duration γ-ray bursts (GRBs) release copious amounts of energy across the entire electromagnetic spectrum, and so provide a window into the process of black hole formation from the collapse of massive stars. Previous early optical observations of even the most exceptional GRBs (990123 and 030329) lacked both the temporal resolution to probe the optical flash in detail and the accuracy needed to trace the transition from the prompt emission within the outflow to external shocks caused by interaction with the progenitor environment. Here we report observations of the extraordinarily bright prompt optical and γ-ray emission of GRB 080319B that provide diagnostics within seconds of its formation, followed by broadband observations of the afterglow decay that continued for weeks. We show that the prompt emission stems from a single physical region, implying an extremely relativistic outflow that propagates within the narrow inner core of a two-component jet.

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.