Swift GRB Afterglows and the Forward-Shock Model

The X-ray light-curves of the GRB afterglows monitored by Swift display one to four phases of power-law decay. In chronological order they are: the burst tail, the ”hump”, the standard decay, and the post jet-break decay. More than half of GRB tails can be identified with the large-angle emission produced during the burst, but arriving at observer later. Several afterglows exhibit a slow, unbroken power-law decay from burst end until 1 day, showing that the forward shock emission is, sometimes, present from the earliest afterglow observations. In fact, the decay of most GRB tails is also consistent with that of the forward-shock emission from a narrow jet (half-angle less than 1o). The X-ray light-curve hump may be due to an increase of the kinetic energy per solid angle of the forward-shock region visible to the observer, caused by either the transfer of energy from some incoming ejecta to the forward shock or by the emergence of the emission from an outflow seen from a location outside the outflow’s opening. However, the correlations among the hump timing, flux, and decay index expected in the latter model are not confirmed by observations. We identify several afterglows whose X-ray light-curves show a second steepening at 0.1–3 day that is consistent with a jet-break. Optical observations for four of them indicate that the X-ray break is achromatic, further strengthening their interpretation as jet-breaks. The decay of 75% of the X-ray afterglows monitored for more than a few days do not exhibit a steepening, implying jet half-angles larger than several degrees. Together with the jet interpretation for the burst tails and the energy injection scenario for the hump, this leads to a radially-structured outflow model, where a narrow, more relativistic, GRB jet precedes a wider, more energetic, afterglow outflow.