Electron acceleration at shocks in merging galaxy clusters

Radio relics are elongated synchrotron radio sources found in the outskirts of galaxy clusters. The relativistic electrons are thought to be accelerated at large-scale merger shocks, and are also considered to be possible sources of ultra-high-energy cosmic rays: they can have energies exceeding $E>10^{18} \\ \text{eV}$.

Particles are accelerated at shocks via Diffusive Shock Acceleration (DSA), gaining energy while crossing a shock multiple times. This process only works from a certain initial energy, which is a more important constraint for electrons than for protons (ions) due to their smaller mass. This is known as the electron injection problem, and it is still a missing puzzle to complete the picture of DSA.

The aim of this project is to study the electron pre-acceleration conditions at galaxy clusters shocks using particle-in-cell simulations. This is the ab-intitio method for kinetic plasma theory, which resolves the microphysics of collisionless shocks. Current studies focus on Stochastic Shock Drift Acceleration (SSDA): an efficient mechanism of electron pre-acceleration. It requires the presence of multiscale turbulence in the shock transition, including shock ripples (shock front corrugations).