Electron Acceleration at Rippled Low Mach Number Shocks in Merging Galaxy Clusters
July 22, 2019
July 02, 2021
Shock waves are ubiquitous in cosmic plasmas wherein they accelerate particles. In particular, X-ray and radio observations of so-called radio relics indicate electron acceleration at large-scale merger shocks in galaxy clusters. These shocks are also candidate sites for ultra-high-energy cosmic ray production. Merger shocks have low Mach numbers and propagate in hot plasmas with plasma beta $\beta\gg 1$. Particle energization and especially electron injection mechanisms are poorly understood in such conditions. Recent studies show that shock drift acceleration (SDA) accompanied by particle-wave interactions can provide electron acceleration, albeit a multi-scale shock structure in the form of ion-scale shock rippling may significantly alter the injection mechanisms. Here we study the effects of the shock rippling with large-scale 2D PIC simulations of low Mach number cluster shocks. We find that the electron acceleration rate increases considerably after the appearance of wave-rippling modes. The main acceleration process is stochastic SDA, in which electrons are confined in the shock transition region by pitch-angle scattering off magnetic turbulence and gain energy from motional electric field. The presence of multi-scale turbulence in the shock is essential for particle energization. Wide-energy non-thermal electron distributions are formed both upstream and downstream of the shock. We show for the first time that the downstream electron spectrum has a power-law form with index $p = 2.4$, in agreement with observations.
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