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 inefficient, converting only a few percent of the kinetic energy into gamma rays. We define ultraefficient internal shocks as those in which the fraction of emitted energy is larger than the fraction of energy given to the radiating electrons at each collision. We show that such a scenario is possible and even plausible. In our model, colliding shells that do not emit all their internal energy are reflected from each other, causing subsequent collisions 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 electrons is ∈_e = 0.1, provided that the shells' Lorentz factor varies between 10 and 10⁴. The numerical temporal profile reflects well the activity of the source that ejects the shells, though numerous collisions take place in this model.