On the existence of a reverse shock in magnetized GRB ejecta

TL;DR

This paper investigates the conditions under which reverse shocks form in magnetized gamma-ray burst ejecta, revealing that strong magnetic fields often suppress reverse shock formation, which explains the scarcity of optical flashes in observations.

Contribution

It derives new criteria for reverse shock existence in arbitrarily magnetized GRB ejecta, considering different density profiles and shell spreading effects.

Findings

Magnetization > 1 generally prevents reverse shock formation.

Shell spreading ensures reverse shocks in low magnetization ejecta.

Magnetic fields explain the lack of optical flashes in GRB afterglows.

Abstract

The role of magnetic fields in gamma-ray burst (GRB) flows remains controversial. The study of the early afterglow phases and, in particular, of the reverse shock dynamics and associated emission offers a promising probe of the magnetization of the ejecta. In this paper, we derive the conditions for the existence of a reverse shock in arbitrarily magnetized ejecta that decelerate and interact with the circumburst medium. Both constant and wind-like density profiles are considered. We show, in contrast to previous estimates, that ejecta with magnetization larger than unity are not crossed by a reverse shock for a large fraction of the parameter space relevant to GRB flows. Allowing for shell spreading, there is always a relativistic or mildly relativistic reverse shock forming in sigma_o < 0.3 ejecta. From this, we conclude that the paucity of optical flashes, believed to be a distinctive signature of a reverse shock, may be explained by the existence of dynamically important magnetic fields in the ejecta.