Search

We present the systematic analysis of the Ultraviolet/Optical Telescope (UVOT) and X-rayTelescope (XRT) light curves for a sample of 26 Swift gamma-ray bursts (GRBs). By comparingthe optical/UV and X-ray light curves, we found that they are remarkably different during thefirst 500 s after the Burst Alert Telescope trigger, while they become more similar during themiddle phase of the afterglow, i.e. between 2000 and 20 000 s.If we take literally the average properties of the sample, we find that the mean temporalindices observed in the optical/UV and X-rays after 500 s are consistent with a forwardshockscenario, under the assumptions that electrons are in the slow cooling regime, theexternal medium is of constant density and the synchrotron cooling frequency is situatedbetween the optical/UV and X-ray observing bands. While this scenario describes well theaveraged observed properties, some individual GRB afterglows require different or additionalassumptions, such as the presence of late energy injection.We show that a chromatic break (a break in the X-ray light curve that is not seen in theoptical) is present in the afterglows of three GRBs and demonstrate evidence for chromaticbreaks in a further four GRBs. The average properties of these breaks cannot be explained interms of the passage of the synchrotron cooling frequency through the observed bands, nora simple change in the external density. It is difficult to reconcile chromatic breaks in termsof a single component outflow and instead, more complex jet structure or additional emissioncomponents are required.

The complex structure of the light curves of Swift GRBs (e.g. superimposed flares and shallow decay) has made their interpretationand that of the blast wave caused by the burst, more difficult than in the pre-Swift era. We aim to constrain the blast wave parameters:electron energy distribution, p, density profile of the circumburst medium, k, and the continued energy injection index, q. We do so bycomparing the observed multi-wavelength light curves and X-ray spectra of a Swift sample to the predictions of the blast wave model.We can successfully interpret all of the bursts in our multi-wavelength sample of 10, except two, within the framework of the blastwave model, and we can estimate with confidence the electron energy distribution index for 6 of the sample. Furthermore we identify jetbreaks in almost half of the bursts. The values of k suggest that the circumburst density profiles are not drawn from only one of theconstant density or wind-like media populations. A statistical analysis of the distribution of p reveals that, even in the most conservativecase of least scatter, the values are not consistent with a single, universal value. This is in agreement with our results for a larger sample ofX-ray only afterglows which we summarise here.

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 ?-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.

The electron energy distribution index, p, is a fundamental parameter of the synchrotron emission from a rangeof astronomical sources. Here we examine one such source of synchrotron emission, gamma-ray burst (GRB)afterglows observed by the Swift satellite. Within the framework of the blast wave model, we examine theconstraints placed on the distribution of p by the observed X-ray spectral indices and parameterize the distribution.We find that the observed distribution of spectral indices are inconsistent with an underlying distribution ofp composed of a single discrete value but consistent with a Gaussian distribution centered at p = 2.36 andhaving a width of 0.59. Furthermore, accepting that the underlying distribution is a Gaussian, we find that themajority ( 94%) of GRB afterglows in our sample have cooling break frequencies less than the X-ray frequency.

The Ultraviolet/Optical Telescope (UVOT) is one of three instruments onboard the Swift observatory.The photometric calibration has been published, and this paper follows up withdetails on other aspects of the calibration including a measurement of the point spreadfunction with an assessment of the orbital variation and the effect on photometry. A correctionfor large-scale variations in sensitivity over the field of view is described, as well asa model of the coincidence loss which is used to assess the coincidence correction in extendedregions. We have provided a correction for the detector distortion and measured theresulting internal astrometric accuracy of the UVOT, also giving the absolute accuracy withrespect to the International Celestial Reference System. We have compiled statistics on thebackground count rates, and discuss the sources of the background, including instrumentalscattered light. In each case, we describe any impact on UVOT measurements, whether anycorrection is applied in the standard pipeline data processing or whether further steps arerecommended.

We present the earliest ultraviolet (UV) spectrum of a gamma-ray burst (GRB) as observedwith the Swift Ultra-Violet/Optical Telescope (UVOT). The GRB 081203A spectrum wasobserved for 50 s with the UV-grism starting 251 s after the Swift-Burst-Alert-Telescope(BAT) trigger. During this time, the GRB was ˜13.4 mag (u filter) and was still rising to itspeak optical brightness. In the UV-grism spectrum, we find a damped Lya line, Lyß and theLyman continuum break at a redshift z = 2.05 ± 0.01. A model fit to the Lyman absorptionimplies a gas column density of log NHI = 22.0 ± 0.1 cm-2, which is typical of GRB hostgalaxies with damped Lya absorbers. This observation of GRB 081203A demonstrates thatfor brighter GRBs (v ˜ 14 mag) with moderate redshift (0.5 < z < 3.5) the UVOT is able toprovide redshifts, and probe for damped Lya absorbers within 4–6 min from the time of theSwift-BAT trigger.