Kinetic and Multiscale Simulations of Particle Acceleration in Astrophysical Shocks

Princeton University, Princeton NJ

Investigators

Abstract

This project will study how particles get accelerated to high speeds in astrophysical sources, focusing on a special mechanism known as Fermi acceleration. Although it is believed that a considerable fraction of the energy in material flowing around pulsars, supernovae, and other strong sources, can be converted into very fast particles, which are identified by the patterns of radiation they give off, the mechanism is not well understood. After this study, the Fermi mechanism will be much better characterized, and may be tightly constrained. Acceleration of particles in collisionless shocks is at the heart of most models of non-thermal phenomena in the Universe. Observations of synchrotron emission suggest that such shocks in pulsar wind nebulae, in jets from active galactic nuclei, in gamma-ray bursts, and in supernova remnants, can convert a significant fraction of the flow energy into relativistic particles with power-law spectra. However, although it is usually invoked as the cause, the conditions for operation of the first-order Fermi mechanism and its efficiency are not understood from first principles. This project will construct a self-consistent theory of shock acceleration by performing large-scale multidimensional particle-in-cell and hybrid simulations of astrophysical collisionless shocks, and study the microphysics of plasma instabilities, particle scattering, and magnetic field generation, necessary to calibrate nonlinear diffusive shock acceleration theory. Specific questions include: 1) the criteria for existence of shock acceleration, and its efficiency; 2) how accelerated particles influence shock structure and evolution; 3) the proton and electron spectra generated in realistic shocks. The tools developed during this work will enable multi-scale modeling of shocks and will derive particle spectra in the nonlinear regime from first principles. This research will provide predictive power to the hypothesis that Fermi acceleration in collisionless shocks is the origin of high-energy cosmic rays and non-thermal particles, and could place tight constraints on the models of magnetization and on the composition of astrophysical outflows. Because Fermi acceleration is a fundamental process in astrophysics, the results will be of value to observers, experimentalists, and theorists studying high-energy astrophysical and cosmological sources, and are even applicable to shocks in the solar system. The findings will guide a new generation of laboratory experiments. The work will involve postdocs, and graduate and undergraduate students, training them in numerical modeling and visualization, and thus preparing them for careers in science and technology fields, where large-scale computing increasingly plays an important role. An interactive plasma physics tutorial will be developed to bring intuitive understanding of these thorny subjects to specialists and the general public alike.