Swift gamma-ray burst after-glows and the forward-shock model

The X-ray light curves of the gamma-ray burst (GRB) afterglows monitored by Swift display one to four phases of power-law decay. In the chronological order they are: the burst tail, the 'hump', the standard decay, and the post-jet-break decay. We compare the decay indices and spectral slopes measured during each phase with the expectations for the forward-shock model to identify the processes which may be at work and to constrain some of their properties. The large-angle emission produced during the burst, but arriving at observer later, is consistent with the GRB tail decay for less than half of bursts. Several afterglows exhibit a slow, unbroken power-law decay from burst end until 1 d, showing that the forward-shock emission is, sometimes, present from the earliest afterglow observations. In fact, the forward-shock synchrotron emission from a very narrow jet (half-angle less than 1 degrees) is consistent with the decay of 75 per cent of GRB tails. The forward-shock inverse-Compton emission from a narrow jet that does not expand sideways also accommodates the decay of 80 per cent of GRB tails. The X-ray light curve hump can be attributed to an increasing kinetic energy per solid angle of the forward-shock region visible to the observer. This increase could be due to the emergence of the emission from an outflow seen from a location outside its opening. However, the correlations among the hump timing, flux, and decay index expected in this model are not confirmed by observations. Thus, the increase in the forward-shock kinetic energy is more likely caused by some incoming ejecta arriving at the shock during the afterglow phase. The jet interpretation for the burst tails and the energy injection scenario for the hump lead to a double-jet outflow structure consisting of a narrow GRB jet which precedes a wider afterglow outflow of lower kinetic energy per solid angle but higher total energy.