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 T₉₀ = 12 s and a fluence of 7.6 × 10^-6 ergs cm^-2 in the 15-150 keV band. With E_γ,iso = 1.26 × 10⁵⁴ 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 α = 1.76. The X-ray energy spectral index was β_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 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°.