Particle Acceleration by Pickup Process Upstream of Relativistic Shocks

TL;DR

This paper investigates particle acceleration mechanisms at relativistic shocks using particle-in-cell simulations, revealing how wakefields and filamentation induce particle pickup and acceleration, with implications for astrophysical phenomena like gamma-ray bursts.

Contribution

It introduces a novel pickup process driven by wakefields and filamentation in relativistic shocks, estimating maximum particle energies and highlighting their astrophysical significance.

Findings

Particles are decoupled from upstream flow by wakefields.

Particles are re-accelerated by motional electric fields.

Maximum Lorentz factors scale with shock parameters, indicating efficient acceleration.

Abstract

Particle acceleration at magnetized purely perpendicular relativistic shocks in electron-ion plasmas are studied by means of two-dimensional particle-in-cell simulations. Magnetized shocks with the upstream bulk Lorentz factor $\gamma_1 \gg 1$ are known to emit intense electromagnetic waves from the shock front, which induce electrostatic plasma waves (wakefield) and transverse filamentary structures in the upstream region via the stimulated/induced Raman scattering and the filamentation instability, respectively. The wakefield and filaments inject a fraction of incoming particles into a particle acceleration process, in which particles are once decoupled from the upstream bulk flow by the wakefield, and are piked up again by the flow. The picked-up particles are accelerated by the motional electric field. The maximum attainable Lorentz factor is estimated as $\gamma_{max,e} \sim \alpha\gamma_1^3$ for electrons and $\gamma_{max,i} \sim (1+m_e\gamma_1/m_i)\gamma_1^2$ for ions, where $\alpha \sim 10$ is determined from our simulation results. $\alpha$ can increase up to $\gamma_1$ for weakly magnetized shock if $\gamma_1$ is sufficiently large. This result indicates that highly relativistic astrophysical shocks such as external shocks of gamma-ray bursts can be an efficient particle accelerator.