Using Combined PIC and MHD to Model Particle Acceleration in Galaxy Cluster Shocks∗)

When clusters of galaxies merge, shocks are formed that are characterized by a low (1-4) sonic Mach number and an Alfvénic Mach number that is typically an order of magnitude higher, giving the shocks a plasma β of approximately 100. The question we seek to answer is to what extent shocks can accelerate particles to relativistic speeds and thereby contribute to the cosmic ray spectrum. We use a combined particle-in-cell and magneto hydrodynamics code, which treats the thermal plasma as a fluid, but uses a kinetic approach to deal with non-thermal particles. This approach is computationally cheaper than the traditional PIC method while preserving the ability to deal with non-thermal particles. Our preliminary results confirm the ability of shocks in the low-Mach, high-β regime that characterizes galaxy cluster merger shocks to accelerate particles depends strongly on the input parameters, which was previously shown with PIC simulations.