The early- and late-time spectral and temporal evolution of GRB 050716

Abstract

We report on a comprehensive set of observations of gamma-ray burst 050716, detected by the Swift satellite and subsequently followed-up rapidly in X-ray, optical and near-infrared (NIR) wavebands. The prompt emission is typical of long-duration bursts, with two peaks in a time interval of T 90 = 68 s (15-350 keV). The prompt emission continues at lower flux levels in the X-ray band, where several smaller flares can be seen on the top of a decaying light curve that exhibits an apparent break around 220 s post-trigger. This temporal break is roughly coincident with a spectral break. The latter can be related to the extrapolated evolution of the break energy in the prompt y -ray emission, and is possibly the manifestation of the peak flux break frequency of the internal shock passing through the observing band. A possible 3o change in the X-ray absorption column is also seen during this time. The late-time afterglow behaviour is relatively standard, with an electron distribution power-law index of p = 2; there is no notable temporal break out to at least 10 d. The broad-band optical/NIR to X-ray spectrum indicates a redshift of z ≥ 2 for this burst, with a host-galaxy extinction value of E B-V ≈ 0.7 that prefers a small magellanic cloud (SMC)-like extinction curve.