Discovery of an extra hard spectral component in the high-energy afterglow emission of GRB 130427A

TL;DR

This paper reports the discovery of an extra hard spectral component in the high-energy afterglow emission of GRB 130427A, likely due to inverse Compton processes, based on detailed Fermi LAT observations.

Contribution

It provides the first clear evidence of an additional hard spectral component in GRB afterglow emission, advancing understanding of high-energy processes in gamma-ray bursts.

Findings

Detection of a hard spectral component in >100 MeV emission

Spectral analysis supports inverse Compton origin

Enhanced understanding of GRB high-energy afterglow

Abstract

The extended high-energy gamma-ray (>100 MeV) emission occurred after the prompt gamma-ray bursts (GRBs) is usually characterized by a single power-law spectrum, which has been explained as the afterglow synchrotron radiation. The afterglow inverse-Compton emission has long been predicted to be able to produce a high-energy component as well, but previous observations have not revealed such a signature clearly, probably due to the small number of >10 GeV photons even for the brightest GRBs known so far. In this Letter, we report on the Fermi Large Area Telescope (LAT) observations of the >100 MeV emission from the very bright and nearby GRB 130427A. We characterize the time-resolved spectra of the GeV emission from the GRB onset to the afterglow phase. By performing time-resolved spectral fits of GRB 130427A, we found a strong evidence of an extra hard spectral component that exists in the extended high-energy emission of this GRB. We argue that this hard component may arise from the afterglow inverse Compton emission.