paper

A Revised Analysis of Gamma Ray Bursts' prompt efficiencies

arXiv:1606.00311 · doi:10.1093/mnras/stw1331

Abstract

The prompt Gamma-Ray Bursts' (GRBs) efficiency is an important clue on the emission mechanism producing the -rays. Previous estimates of the kinetic energy of the blast waves, based on the X-ray afterglow luminosity , suggested that this efficiency is large, with values above 90\% in some cases. This poses a problem to emission mechanisms and in particular to the internal shocks model. These estimates are based, however, on the assumption that the X-ray emitting electrons are fast cooling and that their Inverse Compton (IC) losses are negligible. The observed correlations between (and hence the blast wave energy) and , the isotropic equivalent energy in the prompt emission, has been considered as observational evidence supporting this analysis. It is reasonable that the prompt gamma-ray energy and the blast wave kinetic energy are correlated and the observed correlation corroborates, therefore, the notion is indeed a valid proxy for the latter. Recent findings suggest that the magnetic field in the afterglow shocks is significantly weaker than was earlier thought and its equipartition fraction, , could be as low as or even lower. Motivated by these findings we reconsider the problem, taking now IC cooling into account. We find that the observed correlation is recovered also when IC losses are significant. For small values the blast wave must be more energetic and we find that the corresponding prompt efficiency is significantly smaller than previously thought. For example, for we infer a typical prompt efficiency of .

arXiv comment10 pages, 4 figures. Accepted for publication in MNRAS

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