Energy outflows in γ-ray bursts: discontinuous versus continuous?

Abstract

The recent realization that energy conversion in γ-ray bursts is most likely via internal shocks rather than via external shocks provides additional information about the inner engine: the relativistic flow must be irregular, it must be variable on a short time scale, it must be able to turn off to a very low level and then turn on again, and it must be active for up to a few hundred seconds and possibly much longer. An acceptable model requires that the central engine evolves into a configuration which is stable enough to survive the violent gravitational instabilities associated with the merging/collapse of compact objects, while still keeping enough binding energy to power the burst and in some cases to be active again after some period of quiescence. At present, it is unclear if these separated emission episodes observed in some γ-ray burst light curves are consequences of the same physical process, and if the time separation is due to some intrinsic property of the central source or of its environment. The hypothesis of an intermittent central engine, although intriguing, has to be tested against observations. The feasibility of different models in the production of episodes of quiescence are discussed. Some key theoretical issues are highlighted, along with the types of observations that would determine whether or not the central engine goes dormant for a period of time comparable to the duration of the gaps.