GRB afterglows from long-lived reverse shocks

GRB afterglows are usually associated to the emission of the forward shock propagating in the external medium when the ultra-relativistic outflow is decelerated. However, it is difficult for this model to reproduce some of Swift observations, especially in the early afterglow phase. We discuss a modified scenario [1, 2] where the forward shock is radiatively inefficient in the ultra-relativistic phase, and where the afterglow is produced by the reverse shock propagating within the relativistic outflow itself. To reproduce the observed duration of the afterglow, the reverse shock has to be long-lived, which is possible if the ultra-relativistic part of the outflow is followed by a tail of material with smaller Lorentz factors.