The Swift gamma-ray burst GRB 050422

Abstract

We describe observations of GRB 050422, a Swift-discovered gamma-ray burst. The prompt gamma-ray emission had a T<inf>90</inf> duration of 59 s and was multipeaked, with the main peak occurring at T + 53 s. Swift was able to follow the X-ray afterglow within 100 s of the burst trigger. The X-ray light curve, which shows a steep early decline, can be described by a broken power law with an initial decay slope of α<inf>1</inf> ∼ 5.0, a break time t <inf>b</inf> ∼ 270 s and a post-break decay slope of α<inf>2</inf> ∼ 0.9, when the zero time of the X-ray emission is taken to be the burst trigger time. However, if the zero time is shifted to coincide with the onset of main peak in the gamma-ray light curve then the initial decay slope is shallower with α<inf>1</inf> 3.2. The initial gamma-ray spectrum can be modelled by a power law with a spectral index of β<inf>B</inf> = 0.50 ± 0.19. However, the early time X-ray spectrum is significantly steeper than this and requires a spectral index of β<inf>X</inf> = 2.33 <inf>-0.55</inf>^+0.58. In comparison with other Swift bursts, GRB 050422 was unusually X-ray faint, had a soft X-ray spectrum, and had an unusually steep early X-ray decline. Even so, its behaviour can be accommodated by standard models. The combined BAT/XRT light curve indicates that the initial, steeply declining, X-ray emission is related to the tail of the prompt gamma-ray emission. The shallower decay seen after the break is consistent with the standard afterglow model. © 2006 RAS.