Relativistic Shocks: Particle Acceleration and Magnetization

TL;DR

This review discusses the physics of relativistic shocks, focusing on particle acceleration and magnetic field generation, highlighting recent theoretical and simulation advances, and their implications for high-energy astrophysical phenomena.

Contribution

It provides a comprehensive overview of recent progress in understanding relativistic shock physics through theory and PIC simulations, emphasizing particle acceleration mechanisms and magnetic field generation.

Findings

Efficient particle acceleration occurs in weakly magnetized, quasi-parallel shocks.

Strong magnetic waves are generated ahead of the shock, mediating acceleration.

Accelerated particles exhibit a spectral index of ~2.2 in ultra-relativistic shocks.

Abstract

We review the physics of relativistic shocks, which are often invoked as the sources of non-thermal particles in pulsar wind nebulae (PWNe), gamma-ray bursts (GRBs), and active galactic nuclei (AGN) jets, and as possible sources of ultra-high energy cosmic-rays. We focus on particle acceleration and magnetic field generation, and describe the recent progress in the field driven by theory advances and by the rapid development of particle-in-cell (PIC) simulations. In weakly magnetized or quasi parallel-shocks (where the magnetic field is nearly aligned with the flow), particle acceleration is efficient. The accelerated particles stream ahead of the shock, where they generate strong magnetic waves which in turn scatter the particles back and forth across the shock, mediating their acceleration. In contrast, in strongly magnetized quasi-perpendicular shocks, the efficiencies of both particle acceleration and magnetic field generation are suppressed. Particle acceleration, when efficient, modifies the turbulence around the shock on a long time scale, and the accelerated particles have a characteristic energy spectral index of ~ 2.2 in the ultra-relativistic limit. We discuss how this novel understanding of particle acceleration and magnetic field generation in relativistic shocks can be applied to high-energy astrophysical phenomena, with an emphasis on PWNe and GRB afterglows.