Ultraefficient internal shocks

Gamma-ray bursts are believed to originate from internal shocks that arise in an irregular relativistic wind. The process has been thought to be ineff icient, converting only a few percent of the kinetic energy into gamma rays . We define ultraefficient internal shocks as those in which the fraction o f emitted energy is larger than the fraction of energy given to the radiati ng electrons at each collision. We show that such a scenario is possible an d even plausible. In our model, colliding shells that do not emit all their internal energy are reflected from each other, causing subsequent collisio ns and thereby allowing more energy to be emitted. As an example, we obtain about 60% overall efficiency even if the fraction of energy that goes to e lectrons is epsilon (e) = 0.1, provided that the shells' Lorentz factor var ies between 10 and 10(4). The numerical temporal profile reflects well the activity of the source that ejects the shells, though numerous collisions t ake place in this model.