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

Understand

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.