2007ApJ...664..384D

GRB X-ray light curves display rapid declines followed by a gradual steepening or plateau phase in ≳30% of GRBs in the Swift sample. Treating the standard relativistic blast wave model in a uniform circumburst medium, it is shown 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 timescales ∼10²-10⁴ s after the GRB explosion. While discharging the hadronic energy in the form of ultra-high-energy cosmic-ray neutrals and escaping cosmic-ray ions, the blast wave goes through a strongly radiative phase, causing the steep declines observed with Swift. Following the discharge, the blast wave recovers its adiabatic behavior, forming the observed plateaus or slow declines. These effects are illustrated by calculations of model bolometric light curves. The results show that steep X-ray declines and plateau features occur when GRB sources take place in rather dense media, with n≳10² cm^–3 out to ≳10¹⁷ cm.