Abstract
Context. GRB050721 was detected by Swift and promptly followed-up, in the X-ray by Swift itself and, in the optical band, by the VLT operated, for the first time, in rapid response mode. A multiwavelength monitoring campaign was performed in order to study its afterglow behavior. Aims. We present the analysis of the early and late afterglow emission in both the X-ray and optical bands, as observed by Swift, a robotic telescope, and the VLT. We compare early observations with late afterglow observations obtained with Swift and the VLT in different bands in order to constrain the density of the medium in which the fireball is expanding. Methods. We have analyzed both the X-ray and the optical light curves and compared the spectral energy distribution of the afterglow at two different epochs. Results. We observed an intense rebrightening in the optical band at about one day after the burst which was not seen in the X-ray band. This is the first observation of a GRB afterglow in which a rebrightening is observed in the optical but not in the X-ray band. The lack of detection in X-ray of such a strong rebrightening at lower energies can be described with a variable external density profile. In such a scenario, the combined X-ray and optical observations allow us to derive the matter density at 10^15 cm from the burst. This is about a factor of 10 higher than in the inner region.
Highlights
The Swift Gamma-ray Burst Explorer (Gehrels et al 2004) is currently detecting 2−3 gamma-ray bursts (GRBs) per week, distributing coordinates with very small uncertainties with delays ranging from few seconds to tens of seconds after the GRB event
GRB 050721 was promptly observed in both the X-ray and optical bands and it was accurately monitored for several days after the burst
We have presented a comprehensive multiwavelength study of the afterglow of GRB 050721 based on Rapid Response Mode (RRM) and ToO observation with the Very Large Telescope (VLT), early robotic observations and Swift XRT data
Summary
The Swift Gamma-ray Burst Explorer (Gehrels et al 2004) is currently detecting 2−3 gamma-ray bursts (GRBs) per week, distributing coordinates with very small uncertainties (few arcmin down to several arcsec) with delays ranging from few seconds to tens of seconds after the GRB event. Thanks to its fastpointing capabilities, Swift is able to perform observations of the GRB early afterglow phases, both in the X-ray and ultraviolet/optical bands. The field was observed by the MISTICI collaboration with the ESO-VLT UT2 telescope operated for the first time in rapid response mode (RRM). The VLT observations started 25 min after the GRB and confirmed the presence of the OA within the XRT error circle (Covino et al 2005). Such a prompt identification of the OA allowed a very dense sampling of its light curve at early times, making it one of best examples ever obtained (Testa et al 2005). A detailed study of the XRT and the VLT data is presented and discussed
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