Rapid X-ray Declines in Swift Gamma Ray Burst Lightcurves Explained by A Highly Radiative Blast Wave Phase

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  • 13 June 2006
Abstract
X-ray lightcurves of GRBs show rapid declines some hundreds of seconds after the burst trigger in ~ 30% of GRBs in the Swift sample. Treating the standard blast model in a uniform circumburst medium, we show that if GRBs accelerate ultra-high energy cosmic rays through a Fermi mechanism, then the hadronic component can be rapidly depleted by means of photopion processes on time scales ~10^2 -- 10^4 s after the GRB explosion. While discharging the hadronic energy in the form of ultra-high energy cosmic ray neutrons and escaping cosmic ray ions, the blast wave will go through a strongly radiative phase that drives the mean blast wave Lorentz factor to a radiative asymptote. This process is argued to cause the steep declines observed with Swift in the X-ray light curves of GRBs, implying that GRB sources showing rapid X-ray declines take place in rather dense media, with n >~ 100 cm^{-3} out to >~ 10^{17} cm.

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