On the influence of supra-thermal particle acceleration on the morphology of low-Mach, high-$\beta$ shocks

TL;DR

This study investigates how low-Mach, high-beta shocks in galaxy cluster collisions can accelerate particles, revealing that efficient acceleration occurs above a certain Mach number threshold and significantly influences shock properties.

Contribution

The paper introduces a combined MHD and PIC simulation method to analyze particle acceleration in galaxy cluster shocks across different Mach numbers.

Findings

Shocks with Mach number below 2.25 do not support diffusive shock acceleration.

Efficient particle acceleration occurs at Mach numbers ≥3.

Instabilities in the upstream gas alter shock characteristics and particle injection.

Abstract

When two galaxy clusters encounter each other, the interaction results in a collisionless shock that is characterized by a low (1-4) sonic Mach number, and a high Alfv{\'e}nic Mach number. Our goal is to determine if, and to what extent, such shocks can accelerate particles to sufficient velocities that they can contribute to the cosmic ray spectrum. We combine two different computational methods, magnetohydrodynamics (MHD) and particle-in-cell (PIC) into a single code that allows us to take advantage of the high computational efficiency of MHD while maintaining the ability to model the behaviour of individual non-thermal particles. Using this method, we perform a series of simulations covering the expected parameter space of galaxy cluster collision shocks. Our results show that for shocks with a sonic Mach number below 2.25 no diffusive shock acceleration can take place because of a lack of instabilities in the magnetic field, whereas for shocks with a sonic Mach number $\geq\,3$ the acceleration is efficient and can accelerate particles to relativistic speeds. In the regime between these two extremes, diffusive shock acceleration can occur but is relatively inefficient because of the time- and space-dependent nature of the instabilities. For those shocks that show efficient acceleration, the instabilities in the upstream gas increase to the point where they change the nature of the shock, which, in turn, will influence the particle injection process.