Ultraefficient Internal Shocks

Gamma-ray bursts are believed to originate from internal shocks which 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 ultra efficient 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 which do not emit all their internal energy are reflected from each other and it causes subsequent collisions, 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 $\epsilon_e=0.1$ provided that the shells' Lorentz factor varies between 10 and $10^4$. The numerical temporal profile reflects well the activity of the source which ejects the shells, though numerous collisions take place in this model.