Electron-Positron Pair Loading and the Origin of the Upstream Magnetic Field in GRB Shocks

TL;DR

This paper explores how plasma instabilities driven by electron-positron pair cascades in GRB environments generate turbulent magnetic fields upstream of shocks, potentially explaining the origin of large magnetic fields observed.

Contribution

It demonstrates through 3D particle-in-cell simulations that plasma instabilities from pair cascades produce turbulent magnetic fields, highlighting the role of electromagnetic filamentation instability in GRB shocks.

Findings

Plasma instabilities from pair cascades generate near-equipartition magnetic fields.

Electromagnetic filamentation instability couples pairs and ions effectively.

Results suggest a mechanism for large upstream magnetic fields in GRB shocks.

Abstract

We investigate here the effects of plasma instabilities driven by rapid electron/positron pair cascades, which arise in the environment of GRB sources as a result of back-scattering of a seed fraction of the original spectrum. The injection of electron/positron pairs induces strong streaming motions in the ambient medium. One therefore expects the pair-enriched medium ahead of the forward shock to be strongly sheared on length scales comparable to the radiation front thickness. Using three-dimensional particle-in-cell simulations, we show that plasma instabilities driven by these streaming electron/positron pairs are responsible for the excitation of near-equipartition, turbulent magnetic fields. Our results reveal the importance of the electromagnetic filamentation instability in ensuring an effective coupling between electron/positron pairs and ions, and may help explain the origin of large upstream fields in GRB shocks.