REFRESHED SHOCKS AND AFTERGLOW LONGEVITY IN GAMMA-RAY BURSTS

We consider fireball models where the ejecta have a range of bulk Lore ntz factors, so that the inner (lower Gamma) parts may carry most of t he mass, or even most of the energy. The outer shock and contact disco ntinuity decelerate as the fireball sweeps up external matter. This de celeration allows slower ejecta to catch up, replenishing and reenergi zing the reverse shock and boosting the momentum in the blast wave. In consequence, the energy available to power the afterglow may substant ially exceed that of the burst itself. Such models allow a wide range of possibilities for the afterglow evolution, even in the case of sphe rically symmetric expansion.