Particle Acceleration in Relativistic Magnetized Collisionless ElectronIon Shocks
Abstract
We investigate shock structure and particle acceleration in relativistic magnetized collisionless electronion shocks by means of 2.5dimensional particleincell simulations with iontoelectron mass ratios (m_{i} /m_{e} ) ranging from 16 to 1000. We explore a range of inclination angles between the preshock magnetic field and the shock normal. In "subluminal" shocks, where relativistic particles can escape ahead of the shock along the magnetic field lines, ions are efficiently accelerated via the firstorder Fermi process. The downstream ion spectrum consists of a relativistic Maxwellian and a highenergy powerlaw tail, which contains ~5% of ions and ~30% of ion energy. Its slope is 2.1 ± 0.1. The scattering is provided by shortwavelength nonresonant modes produced by Bell's instability, whose growth is seeded by the current of shockaccelerated ions that propagate ahead of the shock. Upstream electrons enter the shock with lower energy than ions (albeit by only a factor of ~5 Lt m_{i} /m_{e} ), so they are more strongly tied to the field. As a result, only ~1% of the incoming electrons are accelerated at the shock before being advected downstream, where they populate a steep powerlaw tail (with slope 3.5 ± 0.1). For "superluminal" shocks, where relativistic particles cannot outrun the shock along the field, the selfgenerated turbulence is not strong enough to permit efficient Fermi acceleration, and the ion and electron downstream spectra are consistent with thermal distributions. The incoming electrons are heated up to equipartition with ions, due to strong electromagnetic waves emitted by the shock into the upstream. Thus, efficient electron heating (gsim15% of the upstream ion energy) is the universal property of relativistic electronion shocks, but significant nonthermal acceleration of electrons (gsim2% by number, gsim10% by energy, with slope flatter than 2.5) is hard to achieve in magnetized flows and requires weakly magnetized shocks (magnetization σ <~ 10^{3}), where magnetic fields selfgenerated via the Weibel instability are stronger than the background field. These findings place important constraints on the models of gammaray bursts and jets from active galactic nuclei that invoke particle acceleration in relativistic magnetized electronion shocks.
 Publication:

The Astrophysical Journal
 Pub Date:
 January 2011
 DOI:
 10.1088/0004637X/726/2/75
 arXiv:
 arXiv:1009.0024
 Bibcode:
 2011ApJ...726...75S
 Keywords:

 acceleration of particles;
 cosmic rays;
 galaxies: jets;
 gammaray burst: general;
 shock waves;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 24 pages, 20 figures, submitted to ApJ