Prospects for ultra-high-energy particle acceleration at relativistic shocks

• Published in: Monthly notices of the Royal Astronomical Society Vol. 522; no. 4; pp. 4955 - 4962
• Main Authors: Huang, Zhi-Qiu, Reville, Brian, Kirk, John G, Giacinti, Gwenael
• Format: Journal Article
• Language: English
• Published: Oxford University Press 11.05.2023
• Subjects: cosmic rays; acceleration of particles; shock waves
• DOI: 10.1093/mnras/stad1356
• ISSN: 0035-8711

ABSTRACT We study the acceleration of charged particles by ultra-relativistic shocks using test-particle Monte Carlo simulations. Two field configurations are considered: (i) shocks with uniform upstream magnetic field in the plane of the shock, and (ii) shocks in which the upstream magnetic field has a cylindrical geometry. Particles are assumed to diffuse in angle due to frequent non-resonant scattering on small-scale fields. The steady-state distribution of particles’ Lorentz factors is shown to approximately satisfy dN/dγ ∝ γ−2.2 provided the particle motion is scattering dominated on at least one side of the shock. For scattering dominated transport, the acceleration rate scales as tacc ∝ t1/2, though recovers Bohm scaling tacc ∝ t if particles become magnetized on one side of the shock. For uniform field configurations, a limiting energy is reached when particles are magnetized on both sides of the shock. For the cylindrical field configuration, this limit does not apply, and particles of one sign of charge will experience a curvature drift that redirects particles upstream. For the non-resonant scattering model considered, these particles preferentially escape only when they reach the confinement limit determined by the finite system size, and the distribution approaches the escapeless limit dN/dγ ∝ γ−1. The cylindrical field configuration resembles that expected for jets launched by the Blandford & Znajek mechanism, the luminous jets of active galactic nuclei and gamma-ray bursts thus provide favourable sites for the production of ultra-high-energy cosmic rays.