Relativistic Electron Shock Drift Acceleration in Low Mach Number Galaxy Cluster Shocks

TL;DR

This paper investigates how relativistic electron shock drift acceleration can initiate electron acceleration in low Mach number galaxy cluster shocks, potentially explaining observed radio relics.

Contribution

It demonstrates through simulations that thermal electrons can be accelerated via shock drift acceleration, forming a nonthermal population upstream of galaxy cluster shocks.

Findings

Electrons are accelerated and reflected upstream via shock drift acceleration.

Accelerated electrons generate waves that back-scatter them toward the shock.

This mechanism may explain the initial electron acceleration in galaxy cluster shocks.

Abstract

An extreme case of electron shock drift acceleration in low Mach number collisionless shocks is investigated as a plausible mechanism of initial acceleration of relativistic electrons in large-scale shocks in galaxy clusters where upstream plasma temperature is of the order of 10 keV and a degree of magnetization is not too small. One-dimensional electromagnetic full particle simulations reveal that, even though a shock is rather moderate, a part of thermal incoming electrons are accelerated and reflected through relativistic shock drift acceleration and form a local nonthermal population just upstream of the shock. The accelerated electrons can self-generate local coherent waves and further be back-scattered toward the shock by those waves. This may be a scenario for the first stage of the electron shock acceleration occurring at the large-scale shocks in galaxy clusters such as CIZA J2242.8+5301 which has well defined radio relics.