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 ¼ 18:2 about 10 hr after the burst, the optical afterglow was the brightest thus far detected bySwiftand one of the brightest optical afterglows ever seen. The Burst Alert Telescope (BAT) light curves show three fast-rise exponential-decay spikeswithT90 ¼ 12 s and afluence of 7:6 ;10 � 6 ergscm � 2 in the 15‐150 keV band. With E�; iso ¼1:26 ; 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 detectedbytheSwiftUV/Opticaltelescope(UVOT)foralmost3daysandbytheX-RayTelescope(XRT)foraweek after the burst. The X-ray light curve shows a rapidly fading afterglow with a decay index � ¼ 1:76 þ0:15 � 0:07 . The X-ray

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