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

Author: Lara Nava

Abstract: We consider the light curves above 0.1 GeV of 10 GRBs detected by LAT and with measured redshift. In all cases, the emission persists after the prompt has faded away. This extended emission has a similar decay in time (typical index -1.2) among different bursts, while the normalization spans more than 2 decades in luminosity. However, when these luminosities are normalized to the energetics of the prompt emission (estimated in the 1 keV - 10 MeV energy range) all light curves become consistent with having the same normalization, i.e. they cluster. At each given rest frame time the ratio between the LAT luminosity and the prompt energetics is characterized by a small dispersion (less than one decade). We discuss this behavior considering both an external and internal shock scenario for the origin of the LAT emission. In particular, we point out that this result is naturally expected in the standard external shock scenario and it strengthens the interpretation of the GeV emission in terms of afterglow radiation. In this context, the found clustering allow us to derive important informations about the typical values of some parameters of both the afterglow and the prompt emission, such as the efficiency of the particle acceleration process in ultra relativistic external shocks and the overall efficiency of the mechanism producing the prompt.