Particle Injection and Nonthermal Particle Acceleration in Relativistic Magnetic Turbulence*

Relativistic magnetic turbulence has been proposed as a process for producing nonthermal particles in high-energy astrophysics. The particle energization may be contributed by both magnetic reconnection and turbulence, but their interplay is poorly understood. It has been proposed that the parallel electric field due to magnetic reconnection dominates particle acceleration to the low-energy bound of the power-law particle spectrum, but recent studies show that perpendicular electric fields can play an important, if not dominant role. In this study, we carry out fully kinetic particle-in-cell simulations of magnetic turbulence in a pair plasma. We trace a large number of particles and evaluate the contribution of parallel and perpendicular electric fields both during the injection and in the nonthermal acceleration phase that extends to high energy. The injection phase has a growing contribution from the perpendicular electric field with increasing box sizes. We further evaluate important acceleration properties from the kinetic simulations to solve a Fokker-Planck equation for elucidating the physics of nonthermal particle acceleration. We show particle energy distributions are well determined by particle injection, acceleration, and escape processes. The resulting spectral characteristics from the Fokker-Planck approach, including the spectral index and lower and upper bounds of the power-law distribution, agree well with the simulation results. These findings may improve our understanding of non-thermal particles and their emissions in astrophysical plasmas.