Ellison 2013 Monte Carlo Simulations of Nonlinear Particle Acceleration in Parallel Trans-relativistic Shocks

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Title:		Monte Carlo Simulations of Nonlinear Particle Acceleration in Parallel Trans-relativistic Shocks
Authors:		Ellison, Donald C.; Warren, Donald C.; Bykov, Andrei M.
Affiliation:		AA(Physics Department, North Carolina State University, Box 8202, Raleigh, NC 27695, USA don_ellison@ncsu.edu), AB(Physics Department, North Carolina State University, Box 8202, Raleigh, NC 27695, USA ambykov@yahoo.com), AC(Ioffe Institute for Physics and Technology, 194021 St. Petersburg, Russia)
Publication:		The Astrophysical Journal, Volume 776, Issue 1, article id. 46, 17 pp. (2013). (ApJ Homepage)
Publication Date:		10/2013
Origin:		IOP
Astronomy Keywords:		acceleration of particles, cosmic rays, gamma-ray burst: general, shock waves, supernovae: general
DOI:		10.1088/0004-637X/776/1/46
Bibliographic Code:		2013ApJ...776...46E

Abstract

We present results from a Monte Carlo simulation of a parallel collisionless shock undergoing particle acceleration. Our simulation, which contains parameterized scattering and a particular thermal leakage injection model, calculates the feedback between accelerated particles ahead of the shock, which influence the shock precursor and "smooth" the shock, and thermal particle injection. We show that there is a transition between nonrelativistic shocks, where the acceleration efficiency can be extremely high and the nonlinear compression ratio can be substantially greater than the Rankine-Hugoniot value, and fully relativistic shocks, where diffusive shock acceleration is less efficient and the compression ratio remains at the Rankine-Hugoniot value. This transition occurs in the trans-relativistic regime and, for the particular parameters we use, occurs around a shock Lorentz factor γ₀ = 1.5. We also find that nonlinear shock smoothing dramatically reduces the acceleration efficiency presumed to occur with large-angle scattering in ultra-relativistic shocks. Our ability to seamlessly treat the transition from ultra-relativistic to trans-relativistic to nonrelativistic shocks may be important for evolving relativistic systems, such as gamma-ray bursts and Type Ibc supernovae. We expect a substantial evolution of shock accelerated spectra during this transition from soft early on to much harder when the blast-wave shock becomes nonrelativistic.

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