[CRI133] Cosmic ray production by nonlinear diffusive shock acceleration in gamma-ray burst afterglows
The standard analytical model for gamma-ray burst (GRB) afterglows is that they are produced by power-law electron distributions emitting synchrotron, and synchrotron self-Compton, radiation. These electrons, in turn, are produced by highly relativistic shocks sweeping up low-magnetization plasma from the circumburst environment. In this scenario, plasma instabilities develop upstream from the shock and cause two significant departures from the standard model. First, the instabilities transfer incoming energy from ions to electrons, resulting in a large population of both species at energies just less than minimum of the accelerated distribution. Second, particle-in-cell simulations show that injection (into the acceleration process) and acceleration are efficient enough that the presence of accelerated particles becomes dynamically important. We will report on Monte Carlo simulations of diffusive shock acceleration (DSA) taking place at the forward shock of a GRB jet. These simulations include ions and electrons, as well as turbulence-mediated energy transfer between the two species. They also account self-consistently for the interaction between the shock structure and the particles it is accelerating. We expect substantial differences from the standard synchrotron model for afterglows, and we will comment on the feasibility of nonlinear DSA in GRB afterglows as a source of cosmic rays above the ankle.