Kinetic modeling of particle acceleration in astrophysical shock waves.

Due to the low density of astrophysical plasmas, collisionless shocks occur frequently in the cosmos, and are thought to be responsible for the generation of nonthermal particles that span many decades in energy. These particles are observed through synchrotron radiation from astrophysical sources, such as supernova remnants and relativistic jets, or directly as energetic cosmic rays. The main acceleration mechanism is known as "diffusive shock acceleration" and involves particle scattering and diffusion around a shock wave. In the nonlinear stage, shock acceleration couples together the internal structure of the shock with magnetic turbulence generated by accelerated particles, and presents a fascinating self-propagating nonlinear system with multiscale feedbacks. Despite its fundamental importance in astrophysics, the details of shock acceleration mechanism and the conditions for its operation are only now coming to light, particularly due to the advent of ab-initio numerical simulations of collisionless shocks. In this talk I will review the progress in kinetic simulations of shock structure and particle acceleration, focusing on the current understanding of instabilities that lead to magnetic field amplification, the conditions necessary for particle injection into the acceleration process, and the plasma processes that lead to collisionless temperature equilibration between electrons and ions in shocks. These results will be applied to the interpretation of morphologies and radiation spectra of shocks in supernova remnants and galaxy clusters, and to the formulation of effective prescriptions for inclusion of particle acceleration in fluid simulations of astrophysical shocks.