arXiv subjects

J. E. Hill

Publications and source records attributed to J. E. Hill.

42 records · Page 3Linked to original sources

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

The Swift X-ray Telescope

The Swift Gamma-Ray Explorer is designed to make prompt multiwavelength observations of Gamma-Ray Bursts (GRBs) and GRB afterglows. The X-ray Telescope (XRT) enables Swift to determine GRB positions with a few arcseconds accuracy within 100 seconds of the burst onset. The XRT utilizes a mirror set built for JET-X and an XMM/EPIC MOS CCD detector to provide a sensitive broad-band (0.2-10 keV) X-ray imager with effective area of > 120 cm^2 at 1.5 keV, field of view of 23.6 x 23.6 arcminutes, and angular resolution of 18 arcseconds (HPD). The detection sensitivity is 2x10^-14 erg cm^-2 s^-1 in 10^4 seconds. The instrument is designed to provide automated source detection and position reporting within 5 seconds of target acquisition. It can also measure the redshifts of GRBs with Fe line emission or other spectral features. The XRT operates in an auto-exposure mode, adjusting the CCD readout mode automatically to optimize the science return for each frame as the source intensity fades. The XRT will measure spectra and lightcurves of the GRB afterglow beginning about a minute after the burst and will follow each burst for days or weeks.

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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

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

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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arXiv2005

The Discovery of the Optical and Near-IR Afterglows of the First Swift Gamma-Ray Bursts

We present optical and near-infrared searches for afterglow emission from the first four Swift bursts with accurate positions from the X-ray Telescope (XRT). Using telescopes at Las Campanas, Keck, and Palomar observatories we rapidly identified and followed up afterglows for three of the four bursts. The burst positions were also observed with the Very Large Array, but no radio afterglow emission was detected. The optical/NIR afterglows are fainter than about 75% of all afterglows detected to date, with GRB 050126 being the faintest, and were identified thanks to accurate and rapid positions from the XRT and rapid response with >1-m telescopes. This suggests that the fraction of dust-obscured bursts is small, <10% when combined with afterglows localized by the HETE-2 Soft X-ray Camera. The X-ray fluxes are typical of the known population, with the exception of GRB 050126 which has the faintest X-ray afterglow to date (normalized to t=10 hr), and was detected thanks to a response time of only 130 s after the burst. Finally, we find that all three optical/NIR afterglows are located <2 arcsec away from the nominal XRT positions, suggesting that the XRT is capable of delivering highly accurate positions, which will revolutionize afterglow studies.

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