An inverse Compton origin for the 55 GeV photon in the late afterglow of GRB 130907A

TL;DR

This paper reports the detection of a 55 GeV photon in the late afterglow of GRB 130907A, challenging synchrotron origin theories and supporting an inverse Compton mechanism for high-energy emission.

Contribution

It presents evidence that the late high-energy photon likely originates from inverse Compton scattering, not synchrotron radiation, in GRB afterglows.

Findings

A 55 GeV photon was detected 5 hours after the burst.

The photon energy exceeds the maximum synchrotron limit.

Inverse Compton scattering explains the high-energy emission.

Abstract

The extended high-energy gamma-ray (>100 MeV) emission which occurs well after the prompt gamma-ray bursts (GRBs) is usually explained as the afterglow synchrotron radiation. Here we report the analysis of the Fermi Large Area Telescope observations of GRB 130907A. A 55 GeV photon compatible with the position of the burst was found at about 5 hours after the prompt phase. The probability that this photon is associated with GRB 130907A is higher than 99.96%. The energy of this photon exceeds the maximum synchrotron photon energy at this time and its occurrence thus challenges the synchrotron mechanism as the origin for the extended high-energy >10 GeV emission. Modeling of the broad-band spectral energy distribution suggests that such high energy photons can be produced by the synchrotron self-Compton emission of the afterglow.