Particle acceleration in collisionless shocks - Regulated injection and high efficiency

Abstract

Particle acceleration in supernova remnants and radio galaxies proceeds with high efficiency, and at the same time puts out most of the cosmic ray power at moderately relativistic energies. This suggests that there is a regulated injection mechanism that selects only a very small fraction of particles out of the thermal pool, but enough that, when accelerated to moderately relativistic energies, these suprathermal particles contain a significant fraction of the total available energy. It is shown that particle acceleration in shocks has precisely this property because the high-energy particles, by mediating the shock, regulate the acceleration rate of the low-energy ones. A bifurcation in the energy distribution among the particles results; a distinct population of cosmic rays is consistently created out of an initially cold plasma in such a way that the cosmic rays contain more than half the energy in the shocked fluid. Given this model for injection, it becomes possible to study preferential acceleration in detail, and it is shown that partially ionized heavy elements are preferentially accelerated.