Mildly Relativistic Perpendicular Multiple-ion GRB Shocks

Mildly relativistic perpendicular, collisionless multiple-ion gamma-ray burst shocks are analyzed using 2D3V particle-in-cell simulations. A characteristic feature of multiple-ion shocks is alternating maxima of the α particle and the proton densities, at least in the early downstream. Turbulence, shock-drift acceleration, and evidence of stochastic acceleration are observed. We performed simulations with both in-plane (B y ) and out-of-plane (B z ) magnetic fields, as well as in a perpendicular shock setup with φ = 45°, and saw multiple differences: while with B z , the highest-energetic particles mostly gain energy at the beginning of the shock, with B y , particles continue gaining energy and it does not appear that they have reached their final energy level. A larger magnetization σ leads to more high-energetic particles in our simulations. One important quantity for astronomers is the electron acceleration efficiency ϵ e , which is measurable due to synchrotron radiation. This quantity hardly changes when changing the amount of α particles while keeping σ constant. It is, however, noteworthy that ϵ e strongly differs for in-plane and out-of-plane magnetic fields. When looking at the proton and α acceleration efficiency, ϵ p and ϵ α , the energy of α particles always decreases when passing the shock into the downstream, whereas the energy of protons can increase if α particles account for the majority of the ions.