Particle acceleration at relativistic shock waves

TL;DR

This paper reviews recent advances in understanding particle acceleration at relativistic shock waves, emphasizing the role of microturbulence and magnetic turbulence in enabling Fermi acceleration in high-energy astrophysical phenomena.

Contribution

It synthesizes analytical and numerical studies to delineate conditions under which microturbulence facilitates particle acceleration in ultra-relativistic shocks.

Findings

Microturbulence growth is crucial for Fermi acceleration.

The shock magnetization and Lorentz factor determine turbulence excitation.

Conditions for efficient particle acceleration are summarized.

Abstract

Relativistic sources, e.g. gamma-ray bursts, pulsar wind nebulae and powerful active galactic nuclei produce relativistic outflows that lead to the formation of collisionless shock waves, where particle acceleration is thought to take place. Our understanding of relativistic shock acceleration has improved in the past decade, thanks to the combination of analytical studies and high level numerical simulations. In ultra-relativistic shocks, particle acceleration is made difficult by the generically transverse magnetic field and large advection speed of the shocked plasma. Fast growing microturbulence is thus needed to make the Fermi process operative. It is thought, and numerical simulations support that view, that the penetration of supra-thermal particles in the shock precursor generates a magnetic turbulence which in turn produces the scattering process needed for particle acceleration through the Fermi mechanism. Through the comparison of the growth timescale of the microinstabilities in the shock precursor and the precursor crossing timescale, it is possible to delimit in terms of magnetization and shock Lorentz factor the region in which micro-turbulence may be excited, hence whether and how Fermi acceleration is triggered. These findings are summarized here and astrophysical consequences are drawn.