Klein-Nishina effects on the high-energy afterglow emission of gamma-ray bursts

TL;DR

This paper investigates how Klein-Nishina effects influence high-energy gamma-ray afterglow emission in GRBs, revealing suppression of inverse-Compton losses early on and potential explanations for observed rapid decay in certain bursts.

Contribution

It provides a detailed analysis of Klein-Nishina suppression on high-energy synchrotron afterglow emission, highlighting its impact on the temporal decay and brightness of GRB gamma-ray signals.

Findings

Klein-Nishina suppression reduces inverse-Compton losses at early times.

High-energy afterglow emission remains relatively bright initially.

Rapid decay of early high-energy gamma-ray emission can be explained by Klein-Nishina effects.

Abstract

Extended high-energy(>100MeV) gamma-ray emission that lasts much longer than the prompt sub-MeV emission has been detected from quite a few gamma-ray bursts (GRBs) by Fermi Large Area Telescope (LAT) recently. A plausible scenario is that this emission is the afterglow synchrotron emission produced by electrons accelerated in the forward shocks. In this scenario, the electrons that produce synchrotron high-energy emission also undergo inverse-Compton (IC) loss and the IC scattering with the synchrotron photons should be in the Klein-Nishina regime. Here we study effects of the Klein-Nishina scattering on the high-energy synchrotron afterglow emission. We find that, at early times the Klein-Nishina suppression effect on those electrons that produce the high-energy emission is usually strong and therefore their inverse-Compton loss is small with a Compton parameter Y < a few for a wide range of parameter space. This leads to a relatively bright synchrotron afterglow at high energies that can be detected by Fermi LAT. As the Klein-Nishina suppression effect weakens with time, the inverse-Compton loss increases and could dominate over the synchrotron loss in some parameter space. This will lead to a faster temporal decay of the high-energy synchrotron emission than what is predicted by the standard synchrotron model, which may explain the observed rapid decay of the early high-energy gamma-ray emission in GRB090510 and GRB090902B.