The two-component afterglow of Swift GRB 050802

OATES S. R., De Pasquale M., PAGE M. J., BLUSTIN A. J., ZANE S., MCGOWAN K., ...More

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, vol.380, no.1, pp.270-280, 2007 (SCI-Expanded) identifier identifier


This paper investigates GRB 050802, one of the best examples of a Swift gamma-ray burst afterglow that shows a break in the X-ray light curve, while the optical counterpart decays as a single power law. This burst has an optically bright afterglow of 16.5 mag, detected throughout the 170-650 nm spectral range of the Ultraviolet and Optical Telescope (UVOT) onboard Swift. Observations began with the X-ray Telescope and UVOT telescopes 286 s after the initial trigger and continued for 1.2 x 10(6) s. The X-ray light curve consists of three power-law segments: a rise until 420 s, followed by a slow decay with alpha= 0.63 +/- 0.03 until 5000 s, after which, the light curve decays faster with a slope of alpha(3)= 1.59 +/- 0.03. The optical light curve decays as a single power law with alpha(O) = 0.82 +/- 0.03 throughout the observation. The X-ray data on their own are consistent with the break at 5000 s being due to the end of energy injection. Modelling the optical to X-ray spectral energy distribution, we find that the optical afterglow cannot be produced by the same component as the X-ray emission at late times, ruling out a single-component afterglow. We therefore considered two-component jet models and find that the X-ray and optical emission is best reproduced by a model in which both components are energy injected for the duration of the observed afterglow and the X-ray break at 5000 s is due to a jet break in the narrow component. This bright, well-observed burst is likely a guide for interpreting the surprising finding of Swift that bursts seldom display achromatic jet breaks.