Swift observations of GRB 050603:: An afterglow with a steep late-time decay slope

We report the results of Swift observations of the gamma-ray burst GRB 050603. With a V magnitude V 18: 2 about 10 hr after the burst, the optical afterglow was the brightest thus 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 T-90 = 12 s and a fluence of 7.6 x 10(-6) ergs cm(-2) in the 15-150 keV band. With E-gamma,E-iso = 1.26 x 10(54) ergs, it was also one of the most energetic bursts of all times. The Swift spacecraft began observation of the afterglow with the narrow-field instruments about 10 hr 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 alpha = 1.76(-0.07)(+0.15). The X-ray energy spectral index was beta(X) = 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 alpha = 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 degrees-2 degrees.