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

TL;DR

This study analyzes LAT light curves of ten Gamma-Ray Bursts, revealing a universal decay pattern after normalization, supporting the hypothesis that high-energy emission originates from external shock afterglow processes.

Contribution

It demonstrates that normalized LAT light curves of GRBs follow a universal decay, providing evidence for the external shock origin of high-energy emission and constraining key physical parameters.

Findings

Normalized light curves overlap after scaling by $E_{iso}$

LAT emission consistent with synchrotron afterglow model

Parameters $_e$ and $ta_$ are narrowly distributed

Abstract

The physical origin of the >0.1 GeV 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 ($\gtrsim10^2$ seconds) emission, whose luminosity decays in time as a power-law. While the decay rate is similar for all GRBs (i.e. $L_{LAT}\propto t^{-1.2}$), the normalisation spans about two orders of magnitude in luminosity. However, after re-normalising the luminosities to the prompt energetics $E_{iso}$ 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 $E_{iso}$-normalised 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 $\epsilon_e$ and $\eta_\gamma$ (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.