We report the Swift discovery of the nearby long, soft gamma-ray burst GRB100316D, and the subsequent unveiling of its low-redshift host galaxy and associated supernova. We derive the redshift of the ...event to be z= 0.0591 ± 0.0001 and provide accurate astrometry for the gamma-ray burst (GRB) supernova (SN). We study the extremely unusual prompt emission with time-resolved γ-ray to X-ray spectroscopy and find that the spectrum is best modelled with a thermal component in addition to a synchrotron emission component with a low peak energy. The X-ray light curve has a remarkably shallow decay out to at least 800 s. The host is a bright, blue galaxy with a highly disturbed morphology and we use Gemini-South, Very Large Telescope and Hubble Space Telescope observations to measure some of the basic host galaxy properties. We compare and contrast the X-ray emission and host galaxy of GRB100316D to a subsample of GRB-SNe. GRB100316D is unlike the majority of GRB-SNe in its X-ray evolution, but resembles rather GRB060218, and we find that these two events have remarkably similar high energy prompt emission properties. Comparison of the host galaxies of GRB-SNe demonstrates, however, that there is a great diversity in the environments in which GRB-SNe can be found. GRB100316D is an important addition to the currently sparse sample of spectroscopically confirmed GRB-SNe, from which a better understanding of long GRB progenitors and the GRB-SN connection can be gleaned. PUBLICATION ABSTRACT
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 (8t super(-a) with 3 a sub(1) 5), followed by (2) a very shallow decay (0.5 a sub(2) 1.0), and finally (3) a somewhat steeper decay (1 a sub(3) 1.5). These power-law segments are separated by two corresponding break times, t sub(break,1) 500 s and 10 super(3) s t sub(break,2) 10 super(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 (t sub(break,1)) takes place when the forward shock emission becomes dominant, with the intermediate shallow flux decay (a sub(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 (t sub(break,2))- This energy injection increases the energy of the afterglow shock by at least a factor of f 4 and augments the already severe requirements for the efficiency of the prompt gamma-ray emission.
The Early X-Ray Emission from GRBs O’Brien, P. T; Willingale, R; Osborne, J ...
The Astrophysical journal,
08/2006, Volume:
647, Issue:
2
Journal Article
Peer reviewed
Open access
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.
We report the Swift discovery of the nearby long, soft gamma-ray burst GRB 100316D, and the subsequent unveiling of its low-redshift host galaxy and associated supernova. We derive the redshift of ...the event to be z= 0.0591 ± 0.0001 and provide accurate astrometry for the gamma-ray burst (GRB) supernova (SN). We study the extremely unusual prompt emission with time-resolved γ-ray to X-ray spectroscopy and find that the spectrum is best modelled with a thermal component in addition to a synchrotron emission component with a low peak energy. The X-ray light curve has a remarkably shallow decay out to at least 800 s. The host is a bright, blue galaxy with a highly disturbed morphology and we use Gemini-South, Very Large Telescope and Hubble Space Telescope observations to measure some of the basic host galaxy properties. We compare and contrast the X-ray emission and host galaxy of GRB 100316D to a subsample of GRB-SNe. GRB 100316D is unlike the majority of GRB-SNe in its X-ray evolution, but resembles rather GRB 060218, and we find that these two events have remarkably similar high energy prompt emission properties. Comparison of the host galaxies of GRB-SNe demonstrates, however, that there is a great diversity in the environments in which GRB-SNe can be found. GRB 100316D is an important addition to the currently sparse sample of spectroscopically confirmed GRB-SNe, from which a better understanding of long GRB progenitors and the GRB-SN connection can be gleaned.
Gamma-ray bursts (GRBs) come in two classes: long (> 2 s), soft-spectrum bursts and short, hard events. Most progress has been made on understanding the long GRBs, which are typically observed at ...high redshift (z 1) and found in subluminous star-forming host galaxies. They are likely to be produced in core-collapse explosions of massive stars. In contrast, no short GRB had been accurately (< 10″) and rapidly (minutes) located. Here we report the detection of the X-ray afterglow from-and the localization of-the short burst GRB 050509B. Its position on the sky is near a luminous, non-star-forming elliptical galaxy at a redshift of 0.225, which is the location one would expect if the origin of this GRB is through the merger of neutron-star or black-hole binaries. The X-ray afterglow was weak and faded below the detection limit within a few hours; no optical afterglow was detected to stringent limits, explaining the past difficulty in localizing short GRBs.
Aims.We present our analysis of the multiwavelength photometric & spectroscopic observations of GRB 060210 and discuss the results in the overall context of current GRB models. Methods.All available ...optical data underwent a simultaneous temporal fit, while X-ray and γ-ray observations were analysed temporally & spectrally. The results were compared to each other and to possible GRB models. Results.The X-ray afterglow is best described by a smoothly broken power-law with a break at 7.4 h. The late optical afterglow has a well constrained single power-law index which has a value between the two X-ray indices, though it does agree with a single power-law fit to the X-ray. An evolution of the hardness of the high-energy emission is demonstrated and we imply a minimum host extinction from a comparison of the extrapolated X-ray flux to that measured in the optical. Conclusions.We find that the flaring γ-ray and X-ray emission is likely due to internal shocks while the flat optical light curve at that time is due to the external shock. The late afterglow is best explained by a cooling break between the optical and X-rays and continued central engine activity up to the time of the break. The required collimation corrected energy of ~2$\times$1052 erg, while at the high end of the known energy distribution, is not unprecedented.
Optical time series photometry of the short-period magnetic white dwarf + probable brown dwarf binary SDSS J121209.31+013627.7 reveals pulse-like variability in all bands from i′ to u′, increasing ...towards bluer wavelengths and peaking at u′. These modulations are most likely due to a self-eclipsing accretion hot spot on the white dwarf, rotating into view every 88.43 min. This period is commensurate with the Hα radial velocity period of ≈90 min, and consistent with the rotation period of the accretor being equal to the binary orbital period. We combine our observations with other recently reported results to provide an accurate ephemeris. We also detect the system in X-rays with Swift, and estimate the accretion rate at ≈10−13 M⊙ yr−1. We suggest that SDSS J121209.31+013627.7 is most likely a magnetic cataclysmic variable in an extended state of very low accretion, similar to the well-studied polar EF Eri. Alternatively, the putative brown dwarf is not filling its Roche lobe and the system is a detached binary in which the white dwarf is efficiently accreting from the wind of the secondary. However, it is unclear whether an L dwarf wind is strong enough to provide the measured accretion rate. We suggest further observations to distinguish between the Roche lobe overflow and wind accretion scenarios.
Line Searches in Swift X-Ray Spectra Hurkett, C. P; Vaughan, S; Osborne, J. P ...
The Astrophysical journal,
05/2008, Volume:
679, Issue:
1
Journal Article
Peer reviewed
Open access
Prior to the launch of the Swift mission several X-ray line detections were reported in gamma-ray burst afterglow spectra. To date, these pre-Swift era results have not been conclusively confirmed. ...The most contentious issue in this area is the choice of statistical method used to evaluate the significance of these features. In this paper we compare three different methods already extant in the literature for assessing the significance of possible line features and discuss their relative advantages and disadvantages. The methods are demonstrated by application to observations of 40 bursts from the archive of Swift XRT at early times (<a few kiloseconds posttrigger in the rest frame of the burst). Based on this thorough analysis we found no strong evidence for emission lines. For each of the three methods we have determined detection limits for emission- line strengths in bursts with spectral parameters typical of the Swift- era sample. We also discuss the effects of the current calibration status on emission-line detection.
We report the discovery and subsequent multiwavelength afterglow behaviour of the high-redshift (z= 4.27) Gamma Ray Burst (GRB) 050505. This burst is the third most-distant burst, measured by ...spectroscopic redshift, discovered after GRB 000131 (z= 4.50) and GRB 050904 (z= 6.29). GRB 050505 is a long GRB with a multipeaked γ-ray light curve, with a duration of T90= 63 ± 2 s and an inferred isotropic release in γ-rays of ∼ 4.44 × 1053 erg in the 1–104 keV rest-frame energy range. The Swift X-Ray Telescope followed the afterglow for 14 d, detecting two breaks in the light curve at 7.4 +1.5−1.5 and 58.0 +9.9−15.4 ks after the burst trigger. The power-law decay slopes before, between and after these breaks were 0.25+0.16−0.17, 1.17+0.08−0.09 and 1.97+0.27−0.28, respectively. The light curve can also be fitted with a ‘smoothly broken’ power-law model with a break observed at ∼T+ 18.5 ks, with decay slopes of ∼0.4 and ∼1.8, before and after the break, respectively. The X-ray afterglow shows no spectral variation over the course of the Swift observations, being well fitted with a single power law of photon index ∼1.90. This behaviour is expected for the cessation of the continued energization of the interstellar medium shock, followed by a break caused by a jet, either uniform or structured. Neither break is consistent with a cooling break. The spectral energy distribution, indeed, shows the cooling frequency to be below the X-ray but above the optical frequencies. The optical–X-ray spectrum also shows that there is significant X-ray absorption in excess of that due to our Galaxy but very little optical–ultraviolet extinction, with E(B−V) ≈ 0.10 for a Small Magellanic Cloud like extinction curve.
We investigate the origin of the GRB 060912A, which has observational properties that make its classification as either a long or short burst ambiguous. Short-duration gamma-ray bursts (SGRBs) are ...thought to have typically lower energies than long-duration bursts, can be found in galaxies with populations of all ages and are likely to originate from different progenitors to the long-duration bursts. However, it has become clear that duration alone is insufficient to make a distinction between the two populations in many cases, leading to a desire to find additional discriminators of burst type. GRB 060912A had a duration of 6 s and occurred only ∼10 arcsec from a bright, low-redshift (z= 0.0936) elliptical galaxy, suggesting that this may have been the host, which would favour it being a short burst. However, our deep optical imaging and spectroscopy of the location of GRB 060912A using the Very Large Telescope (VLT) shows that GRB 060912A more likely originates in a distant star-forming galaxy at z= 0.937, and is most likely a long burst. This demonstrates the risk in identifying bright, nearby galaxies as the hosts of given gamma-ray bursts (GRBs) without further supporting evidence. Further, it implies that, in the absence of secure identifications, ‘host’ type, or more broadly discriminators that rely on galaxy redshifts, may not be good indicators of the true nature of any given GRB.
PDF
