e± Pair Loading and the Origin of the Upstream Magnetic Field in GRB Shocks

We investigate here the effects of plasma instabilities driven by rapid e±-pair cascades, which arise in the environment of γ-ray burst (GRB) sources as a result of back-scattering of a seed fraction of the original spectrum. The injection of e± 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 e± 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 e± pairs and ions, and may help explain the origin of large upstream fields in GRB shocks.