A New Paradigm for Gamma Ray Bursts:Long Term Accretion Rate Modulation by an External Accretion Disk

We present a new way of looking at the very long term evolution of GRBs in which the disk of material surrounding the putative black hole powering the GRB jet modulates the mass flow, and hence efficacy of the process that extracts rotational energy from the black hole. The pre-Swift paradigm of achromatic, shallow-to-steep breaks in the long term GRB light curves has not been borne out by detailed Swift data amassed in the past several years. We argue that, given the initial existence of a fall-back disk near the progenitor, an unavoidable consequence will be the formation of an external disk whose outer edge continually moves to larger radii due to angular momentum transport and lack of a confining torque. The mass reservoir at large radii moves outward with time and gives a natural power law decay to the GRB light curves. In this model, the four different canonical power law decay segments in the GRB identified by Zhang et al. represent different physical states of the accretion disk.