Collisionless Weibel shocks and electron acceleration in gamma-ray bursts

TL;DR

This paper uses 3D particle-in-cell simulations to analyze collisionless shocks in gamma-ray bursts, revealing electron acceleration mechanisms driven by ion Weibel instability and electromagnetic fields.

Contribution

It provides a self-consistent simulation-based analysis of shock structure, electromagnetic fields, and electron acceleration in gamma-ray burst environments, highlighting the role of ion filaments.

Findings

Electrons are heated to energies proportional to the square root of magnetic energy.

Electrons are trapped and reflected by electromagnetic fields in the shock region.

Electrons are accelerated via drift motion and shock surfing acceleration mechanisms.

Abstract

A study of collisionless external shocks in gamma-ray bursts is presented. The shock structure, electromagnetic fields, and process of electron acceleration are assessed by performing a self-consistent 3D particle-in-cell (PIC) simulation. In accordance with hydrodynamic shock systems, the shock consists of a reverse shock (RS) and forward shock (FS) separated by a contact discontinuity (CD). The development and structure are controlled by the ion Weibel instability. The ion filaments are sources of strong transverse electromagnetic fields at both sides of the double shock structure over a length of 30 - 100 ion skin depths. Electrons are heated up to a maximum energy $\epsilon_{\rm ele}\approx \sqrt{\epsilon_{\rm b}}$, where $\epsilon$ is the energy normalized to the total incoming energy. Jet electrons are trapped in the RS transition region due to the presence of an ambipolar electric field and reflection by the strong transverse magnetic fields in the shocked region. In a process similar to shock surfing acceleration (SSA) for ions, electrons experience drift motion and acceleration by ion filament transverse electric fields in the plane perpendicular to the shock propagation direction. Ultimately accelerated jet electrons are convected back into the upstream.