Low-energy Injection and Nonthermal Particle Acceleration in Relativistic Magnetic Turbulence

TL;DR

This study uses 2D kinetic simulations to explore how relativistic magnetic turbulence accelerates particles, revealing the dominant electric field contributions during different energization phases and how domain size influences particle injection and acceleration.

Contribution

It provides new insights into the roles of parallel and perpendicular electric fields in particle acceleration within relativistic turbulence, highlighting the importance of domain size effects.

Findings

Injection energy converges to ~10m_ec^2 with increasing domain size.

Perpendicular electric fields increasingly contribute to injection as domain size grows.

Perpendicular electric fields dominate energization of high-energy particles.

Abstract

Relativistic magnetic turbulence has been proposed as a process for producing nonthermal particles in high-energy astrophysics. Particle energization may be contributed by both magnetic reconnection and turbulent fluctuations, but their interplay is poorly understood. It has been suggested that during magnetic reconnection the parallel electric field dominates particle acceleration up to the lower bound of the power-law particle spectrum, but recent studies show that electric fields perpendicular to magnetic field can play an important, if not dominant role. In this study, we carry out 2D fully kinetic particle-in-cell simulations of magnetically dominated decaying turbulence in a relativistic pair plasma. For a fixed magnetization parameter $\sigma_0=20$, we find that the injection energy ${\varepsilon}_{\rm inj}$ converges with increasing domain size to ${\varepsilon}_{\rm inj}\simeq 10m_ec^2$. In contrast, the power-law index, the cut-off energy, and the power-law extent increase steadily with domain size. We trace a large number of particles and evaluate the contributions of the work done by the parallel ($W_\parallel$) and perpendicular ($W_\perp$) electric fields during both the injection phase and the post-injection phase. We find that during the injection phase, the $W_\perp$ contribution increases with domain size, suggesting that it may eventually dominate injection for a sufficiently large domain. In contrast, both components contribute equally during the post-injection phase, insensitive to the domain size. For high energy (${\varepsilon}\gg{\varepsilon}_{\rm inj}$) particles, $W_\perp$ dominates the subsequent energization. These findings may improve our understanding of nonthermal particles and their emissions in astrophysical plasmas.