<i>Swift</i>Panchromatic Observations of the Bright Gamma‐Ray Burst GRB 050525a

Abstract

The bright gamma-ray burst GRB050525a has been detected with the Swift\nobservatory, providing unique multiwavelength coverage from the very earliest\nphases of the burst. The X-ray and optical/UV afterglow decay light curves both\nexhibit a steeper slope ~0.15 days after the burst, indicative of a jet break.\nThis jet break time combined with the total gamma-ray energy of the burst\nconstrains the opening angle of the jet to be 3.2 degrees. We derive an\nempirical `time-lag' redshift from the BAT data of z_hat = 0.69 +/- 0.02, in\ngood agreement with the spectroscopic redshift of 0.61.\n Prior to the jet break, the X-ray data can be modelled by a simple power law\nwith index alpha = -1.2. However after 300 s the X-ray flux brightens by about\n30% compared to the power-law fit. The optical/UV data have a more complex\ndecay, with evidence of a rapidly falling reverse shock component that\ndominates in the first minute or so, giving way to a flatter forward shock\ncomponent at later times.\n The multiwavelength X-ray/UV/Optical spectrum of the afterglow shows evidence\nfor migration of the electron cooling frequency through the optical range\nwithin 25000 s. The measured temporal decay and spectral indices in the X-ray\nand optical/UV regimes compare favourably with the standard fireball model for\nGamma-ray bursts assuming expansion into a constant density interstellar\nmedium.\n