On the Contribution of low-mach, high-beta Shocks to the Cosmic Ray Spectrum

Astrophysical shocks accelerate particles through the Fermi acceleration process, which involves a charged particle repeatedly crossing the shock after being reflected by the local magnetic field and gaining momentum. Eventually, the particles reach relativistic speeds and can be observed as cosmic rays. This is a self self-sustaining interaction because the presence of non-thermal particles in the shock-region causes instabilities in the magnetic field, which in turn allow the magnetic field to reflect the particles. This process has been studied extensively in the case of high-Mach, low-β shocks, such as those that are found in stellar wind collisions and supernovae. However, there are astrophysical shocks, such as those that occur in colliding galaxy clusters, that are characterized by a low sonic Mach number, combined with a high plasma-beta. So far, these shocks have been largely neglected, and little is known about their ability to accelerate particles. Using a combined PIC-MHD code, we have performed a series of numerical simulations of low-Mach, high-beta shocks, to investigate the interaction between the particles and the magnetic field under such conditions. We find that even low-Mach shocks are capable of accelerating charged particles. However, due to the behaviour of the magnetic field, the process tends to be relatively inefficient, reducing the effective contribution to the cosmic ray spectrum. Furthermore, the interaction tends to radically change the nature of the shock itself, which indicates that further study is required to quantify the shocks' long-term behaviour.