X-ray flares, neutrino-cooled discs and the dynamics of late accretion in gamma-ray burst engines

Abstract

Abstract We compute the average luminosity of X-ray flares as a function of time, for a sample of 10 long-duration gamma-ray burst (GRB) afterglows. The mean luminosity, averaged over a time-scale longer than the duration of the individual flares, declines as a power law in time with index approximately -1.5. We elaborate on the properties of the central engine that can produce such a decline. Assuming that the engine is an accreting compact object, and for a standard conversion factor between accretion rate and jet luminosity, the switch between a neutrino-cooled thin disc and a non-cooled thick disc takes place at the transition from the prompt to the flaring phase. We discuss the implications of this coincidence under different scenarios for the powering of the GRB outflow. We also show that the interaction of the outflow with the envelope of the progenitor star cannot produce flares out of a continuous relativistic flow, and conclude that it is the dynamics of the disc or the jet launching mechanism that generates an intrinsically unsteady outflow on time-scales much longer than the dynamical time-scale of the system. This is consistent with the fact that X-ray flares are observed in short-duration GRBs as well as in long-duration ones.