SwiftPanchromatic Observations of the Bright Gamma‐Ray Burst GRB 050525a

The bright gamma-ray burst GRB050525a has been detected with the Swiftobservatory, providing unique multiwavelength coverage from the very earliestphases of the burst. The X-ray and optical/UV afterglow decay light curves bothexhibit a steeper slope ~0.15 days after the burst, indicative of a jet break.This jet break time combined with the total gamma-ray energy of the burstconstrains the opening angle of the jet to be 3.2 degrees. We derive anempirical `time-lag' redshift from the BAT data of z_hat = 0.69 +/- 0.02, ingood agreement with the spectroscopic redshift of 0.61. Prior to the jet break, the X-ray data can be modelled by a simple power lawwith index alpha = -1.2. However after 300 s the X-ray flux brightens by about30% compared to the power-law fit. The optical/UV data have a more complexdecay, with evidence of a rapidly falling reverse shock component thatdominates in the first minute or so, giving way to a flatter forward shockcomponent at later times. The multiwavelength X-ray/UV/Optical spectrum of the afterglow shows evidencefor migration of the electron cooling frequency through the optical rangewithin 25000 s. The measured temporal decay and spectral indices in the X-rayand optical/UV regimes compare favourably with the standard fireball model forGamma-ray bursts assuming expansion into a constant density interstellarmedium.Comment: 31 pages, 7 figures, referee comments implemented, typo corrected in author list, accepted by Ap