Swift Panchromatic Observations of the Bright Gamma‐Ray Burst GRB 050525a

Abstract

The bright gamma-ray burst GRB 050525a has been detected with the Swift observatory, providing unique multi- wavelength coverage fromthe very earliest phases of the burst. TheX-ray and optical/UVafterglow decay light curves both exhibit 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 burst constrains the opening angle of the jet to be 3N2.We derive an empirical ‘‘time- lag’’ redshift from theBAT data of ˆ z ¼ 0:69 ? 0:02, in good agreement with the spectroscopic redshift of 0.61. Prior to the jet break, the X-ray data can be modeled by a simple power law with index ? ¼?1:2. However, after 300 s the X-ray flux brightens by about 30%compared to the power-law fit. The optical/UVdata have a more complex decay, with evidence of a rapidly falling reverse shock component that dominates in the first minute or so, giving way to a flatter forward shock component at later times. The multiwavelength X-ray/UV/optical spectrum of the afterglow shows evidence for migration of the electron cooling frequency through the optical range within 25,000 s. The mea- sured temporal decay and spectral indexes in the X-ray and optical/UV regimes compare favorablywith the standard fireball model for gamma-ray bursts assuming expansion into a constant-density interstellar medium.