Clustering of LAT light curves: a clue to the origin of high-energy emission in Gamma-Ray Bursts

TLDR: In this paper, the authors consider the GeV light curves of ten Gamma-Ray Bursts (GRBs) with measured redshift detected by the Fermi-LAT, and they consider the scenario in which the temporally extended LAT emission is dominated by synchrotron radiation from electrons accelerated at forward external shock.

Abstract: The physical origin of the > 0.1GeV emission detected from Gamma-Ray Bursts (GRBs) by the Fermi satellite has not yet been completely understood. In this work we consider the GeV light curves of ten GRBs with measured redshift detected by the Fermi-LAT. These light curves are characterised by a long-lived (& 10 2 seconds) emission, whose luminosity decays in time as a power-law. While the decay rate is similar for all GRBs (i.e. LLAT / t 1.2 ), the normalisation spans about two orders of magnitude in luminosity. However, after re-normalising the luminosities to the prompt energetics Eprompt the light curves overlap. We consider the scenario in which the temporally extended LAT emission is dominated by synchrotron radiation from electrons accelerated at the forward external shock. According to this model, at high-energies (i.e. above the typical synchrotron frequencies) a small dispersion of the Epromptnormalised light curves is expected. The fact that the LAT temporally extended emission follows this behaviour reinforces its interpretation in terms of afterglow radiation from external shocks. Assuming this scenario, we argue that the parameters ǫe and η (i.e., the fraction of shock-dissipated energy gained by the electrons, and the efficiency of the mechanism producing the prompt radiation, respectively) must be narrowly distributed.