A revised analysis of gamma-ray bursts' prompt efficiencies

Paz Beniamini, Lara Nava, Tsvi Piran

Research output: Contribution to journalArticlepeer-review


The prompt gamma-ray bursts' (GRBs) efficiency is an important clue on the emission mechanism producing the γ-rays. Previous estimates of the kinetic energy of the blast waves, based on the X-ray afterglow luminosity LX, suggested that this efficiency is large, with values above 90 per cent 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 LX (and hence the blast wave energy) and Eγ,iso, 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 LX 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, εB, could be as low as 10-4 or even lower. Motivated by these findings we reconsider the problem, taking now IC cooling into account. We find that the observed LX - Eγ,iso correlation is recovered also when IC losses are significant. For small εB 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 εB ~ 10-4 we infer a typical prompt efficiency of ~15 per cent.

Original languageEnglish
Pages (from-to)51-59
Number of pages9
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - 1 Sep 2016
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


  • Gamma-ray burst: general


Dive into the research topics of 'A revised analysis of gamma-ray bursts' prompt efficiencies'. Together they form a unique fingerprint.

Cite this