Particle Acceleration in Astrophysical Collisionless Shocks

Princeton University, Princeton NJ

Investigators

Abstract

AST-0807381 Spitkovsky Acceleration of particles in collisionless shocks is at the heart of most models of nonthermal phenomena in the Universe, including pulsar wind nebulae, active galactic nuclei, gamma-ray bursts, and supernova remnants. The conversion of flow energy into relativistic particles is usually attributed to the first-order Fermi mechanism, but this is not well understood. This project will construct a self-consistent theory of shock acceleration by performing the first large-scale three-dimensional particle-in-cell (PIC) simulations of astrophysical collisionless shocks. The PIC method is the most fundamental ab initio approach to plasma dynamics that is able to resolve directly the complex microphysics of plasma instabilities, particle scattering, and magnetic field generation near shocks. Specific questions to be addressed include: 1) Do collisionless shocks generate significant magnetic fields that can survive far downstream? 2) What fraction of shocked particles is injected into the Fermi mechanism? 3) What is the effect of high-energy particles on shock hydrodynamics and magnetic field amplification? 4) What mechanisms cause equilibration between particle species in shocks? This study will be a definitive test of the prevailing hypothesis that Fermi acceleration in collisionless shocks is the origin of high energy cosmic rays and nonthermal particles in a variety of astrophysical sources. The results will be of broad significance to observers, experimentalists, and theorists involved with experiments to study high-energy astrophysical and cosmological sources. The research integrates research and education by involving graduate and undergraduate students, and postdocs, training in numerical modeling and visualization of multiscale systems. An offshoot will be a comprehensive web-based introduction to the physics of collisionless shocks. In addition, a new version of the massively-parallel PIC code TRISTAN-MP and its analysis tools will be publicly released and supported on the web site.