Swift Panchromatic Observations of the Bright Gamma-Ray Burst GRB 050525a
Sakamoto, T.; Campana, S.; Tagliaferri, G.; Chincarini, G.; Cucchiara, A.; White, N. E.; Hurley, K. C.; Page, M. J.; Moretti, A.; Gehrels, N.; Giommi, P.; Schady, P.; Roming, P.; Chester, M. M.; Goad, M.; O'Brien, P. T.; Romano, P.; La Parola, V.; Kennea, J. A.; Grupe, D.; Mineo, T.; Perri, M.; Rosen, S.; Osborne, J. P.; Burrows, D. N.; Mason, K. O.; Zhang, B.; Cummings, J. R.; Wells, A. A.; Barthelmy, S.; de Pasquale, M.; Holland, S. T.; Angelini, L.; Cominsky, L. R.; Palmer, D. M.; Capalbi, M.; Beardmore, A.; Hill, J. E.; Mészáros, P.; Barbier, L.; Fenimore, E. E.; Sato, G.; Markwardt, C. B.; Tueller, J.; Kumar, P.; Krimm, H. A.; Landsman, W.; Marshall, F. E.; Gronwall, C.; Boyd, P.; Still, M.; Page, K.; Blustin, A. J.; Rhoads, J.; Parsons, A. M.; Band, D.; Breeveld, A.; Nousek, J.; Hunsberger, S.; Cannizzo, J.; Poole, T.; Hinshaw, D.; Ivanushkina, M.; McGowan, K.; Morgan, A.
United Kingdom, United States, Italy, Japan
Abstract
The bright gamma-ray burst GRB 050525a has been detected with the Swift observatory, providing unique multiwavelength coverage from the very earliest phases of the burst. The X-ray and optical/UV afterglow 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 3.2d. We derive an empirical ``time-lag'' redshift from the BAT data of ẑ=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 α=-1.2. However, after 300 s the X-ray flux brightens by about 30% compared to the power-law fit. The optical/UV data 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 measured temporal decay and spectral indexes in the X-ray and optical/UV regimes compare favorably with the standard fireball model for gamma-ray bursts assuming expansion into a constant-density interstellar medium.