On the origin of >10 GeV photons in gamma-ray burst afterglows

TL;DR

This paper investigates the maximum energy of synchrotron photons in gamma-ray burst afterglows, revealing that synchrotron emission cannot account for >10 GeV photons observed after the prompt phase, suggesting alternative mechanisms.

Contribution

It provides a detailed analysis of the maximum synchrotron photon energy considering microturbulence effects, showing the energy decreases rapidly and cannot explain >10 GeV photons.

Findings

Maximum synchrotron photon energy is a few GeV at 100 s after the burst.

Synchrotron mechanism cannot produce 10-100 GeV photons observed after the prompt phase.

Alternative processes like synchrotron self-Compton or external inverse-Compton are likely responsible.

Abstract

Fermi/LAT has detected long-lasting high-energy photons (>100 MeV) from gamma-ray bursts (GRBs), with the highest energy photons reaching about 100 GeV. One proposed scenario is that they are produced by high-energy electrons accelerated in GRB forward shocks via synchrotron radiation. We study the maximum synchrotron photon energy in this scenario, considering the properties of the microturbluence magnetic fields behind the shock, as revealed by recent Particle-in-Cell simulations and theoretical analyses of relativistic collisionless shocks. Due to the small-scale nature of the micro-turbulent magnetic field, the Bohm acceleration approximation breaks down at such high energies. This effect leads to a typical maximum synchrotron photon of a few GeV at 100 s after the burst and this maximum synchrotron photon energy decreases quickly with time. We show that the fast decrease of the maximum synchrotron photon energy leads to a fast decay of the synchrotron flux. The 10-100 GeV photons detected after the prompt phase can not be produced by the synchrotron mechanism. They could originate from the synchrotron self-Compton emission of the early afterglow if the circum-burst density is sufficiently large, or from the external inverse-Compton process in the presence of central X-ray emission, such as X-ray flares and prompt high-latitude X-ray emission.