The internal-external GRB-afterglow model

Most current GRB models are based on the conversion, via shocks, of the energy of an ultra-relativistic flow to the observed radiation. We show that external shocks cannot produce the highly variable temporal structure observed in most GRBs. Internal shocks, on the other hand can produce this structure, provided that the inner engine that produce the relativistic flow generates an unsteady and irregular "wind". However, internal shocks can convert only a fraction of the total energy to radiation. Most of the kinetic energy of the flow must be eventually dissipated via external shocks. We suggest that this is the origin of the observed afterglow. According to this Internal-external model the GRB and afterglow are not produced by the same process and hence they do not scale directly to each other. Furthermore; under certain conditions the initial phase of the afterglow might radiate in gamma rays and in these cases this signal will overlap as a smooth component with the variable signal produced by the internal shocks, leading to a combined scenario of the GRB itself.