Swift Observations of GRB050603: An afterglow with a steep late time decay slope

We report the results of Swift observations of the Gamma Ray Burst GRB050603. With a V magnitude V=18.2 about 10 hours after the burst the optical afterglow was the brightest so far detected by Swift and one of the brightest optical afterglows ever seen. The Burst Alert Telescope (BAT) light curves show three fast-rise-exponential-decay spikes with T90=12s and a fluence of 7.6×10 ergs cm in the 15-150 keV band. With an Eγ,iso = 1.26× 10 54 ergs it was also one of the most energetic bursts of all times. The Swift spacecraft began observing of the afterglow with the narrow-field instruments about 10 hours after the detection of the burst. The burst was bright enough to be detected by the Swift UV/Optical telescope (UVOT) for almost 3 days and by the X-ray Telescope (XRT) for a week after the burst. The X-ray light curve shows a rapidly fading afterglow with a decay index α=1.76 −0.07. The X-ray energy spectral index was βX=0.71±0.10 with the column density in agreement with the Galactic value. The spectral analysis does not show an obvious change in the X-ray spectral slope over time. The optical UVOT light curve decays with a slope of α=1.8±0.2. The steepness and the similarity of the optical and X-ray decay rates suggest that the afterglow was observed after the jet break. We estimate a jet opening angle of about 1-2. Subject headings: GRBs:individual(GRB 050603) Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802 NASA Goddard Space Flight Center, Greenbelt, MD 20771 Department of Physics, University of Nevada, Las Vegas, NV 89154 ASI Science Data Center, via G. Galilei, I-00044 Frascati (Roma), Italy Universities Space Research Association, Seabrook, MD Department of Physics, Pennsylvania State University, University Park, PA 16802 Department of Physics & Astronomy, University of Leicester, Leicester, LE1 7R, UK Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, U.K.