Ultraefficient Internal Shocks

Gamma-ray bursts are believed to originate from internal shocks which arisein an irregular relativistic wind. The process has been thought to beinefficient, converting only a few percent of the kinetic energy intogamma-rays. We define ultra efficient internal shocks as those in which thefraction of emitted energy is larger than the fraction of energy given to theradiating 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 fromeach other and it causes subsequent collisions, allowing more energy to beemitted. As an example, we obtain about 60% overall efficiency even if thefraction of energy that goes to electrons is $\epsilon_e=0.1$ provided that theshells' Lorentz factor varies between 10 and $10^4$. The numerical temporal profile reflects well the activity of the source whichejects the shells, though numerous collisions take place in this model.Comment: 16 pages, 6 figures, accepted in Ap