Particle acceleration at ultrarelativistic, perpendicular shock fronts

TL;DR

This paper demonstrates that ultrarelativistic, perpendicular shock fronts can efficiently accelerate particles and produce a power-law spectrum, challenging previous beliefs about the role of magnetic turbulence in such processes.

Contribution

It introduces a novel eigenfunction expansion method combined with Monte-Carlo simulations to show effective particle acceleration at perpendicular shocks without large turbulence.

Findings

Ultrarelativistic perpendicular shocks produce a power-law spectrum with index 4.17.

Particle acceleration is effective even with a uniform downstream magnetic field.

Results challenge the belief that turbulence is necessary for acceleration in such shocks.

Abstract

Using an eigenfunction expansion to solve the transport equation, complemented by Monte-Carlo simulations, we show that ultrarelativistic shocks can be effective particle accelerators even when they fail to produce large amplitude turbulence in the downstream plasma. This finding contradicts the widely held belief that a uniform downstream magnetic field perpendicular to the shock normal inhibits acceleration by the first order Fermi process. In the ultrarelativistic limit, we find a stationary power-law particle spectrum of index s=4.17 for these shocks, close to that predicted for a strictly parallel shock.