Particle acceleration in non-uniform mach number shocks with particle-in-cell simulation

Abstract

Radio relics detected in the outskirts of galaxy clusters provide direct evidence of Diffusive Shock Acceleration (DSA) in the hot intracluster medium (ICM). These relics typically exhibit sonic Mach numbers (𝑀𝑠) less than 4. Recent studies suggest that the pre-acceleration and injection of electrons may occur only at supercritical shocks with 𝑀𝑠 greater than 2.3 in high-beta environments in the ICM. However, some observed radio relics display subcritical shocks with 𝑀𝑠 less than 2.3, contradicting previous simulation results. One plausible explanation for this inconsistency is the presence of a non- uniform shock surface characterized by varying 𝑀𝑠 and obliquity angles. To explore DSA at such weak shocks, we conducted Particle-in-Cell (PIC) simulations using a sinusoidal plasma beam profile to generate a turbulent shock surface with variable obliquity angles. This thesis presents some of the first results from these simulations and shows how they compare to single Mach number shocks in a uniform background. It also discusses what these results mean for the idea that subcritical shocks could generate radio relics.