Diffusive shock acceleration of cosmic rays in low-Mach galaxy cluster shocks.

Astrophysical shocks are known to accelerate particles to high relativistic velocities. This process requires the particles to repeatedly cross the shock, a process that can only occur if the particle is reflected by the local magnetic field. Such particles are observed on Earth as cosmic rays. This phenomenon has been studied in considerable detail for high-Mach shocks, such as the shocks that occur in colliding stellar winds and supernova explosions, but remains relatively unexplored for low-Mach shocks, such as the shocks of colliding clusters of galaxies. Recent simulations using the particle-in-cell (PIC) method have shown that, depending on the exact Mach number, even low-Mach shocks can accelerate charged particles to the point where they start to deviate from the thermal velocity distribution. However, the computationally intensive nature of the PIC calculations makes it difficult to continue the simulations to determine whether the particles can reach relativistic speeds. We now present new simulations, using a combined PIC and magnetohydrodynamics (MHD) technique. This model, which takes advantage of the computational efficiency of MHD, allows us to simulate a much larger physical volume and study the behaviour of the particles over a longer period of time in order to determine to what extent the acceleration process continues and whether these shocks are capable of contributing to the cosmic ray spectrum.