The interplay of magnetically-dominated turbulence and magnetic reconnection in producing nonthermal particles

Abstract: Magnetized turbulence and magnetic reconnection are often invoked to explain the nonthermal emission observed from a wide variety of astrophysical sources. By means of fully-kinetic 2D and 3D PIC simulations, we investigate the interplay between turbulence and reconnection in generating nonthermal particles in magnetically-dominated pair plasmas. A generic by-product of the turbulence evolution is the generation of a nonthermal particle spectrum with a power-law energy range. The power-law slope $p$ is harder for larger magnetizations and stronger turbulence fluctuations, and it can be as hard as $p < 2$. The Larmor radius of particles at the high-energy cutoff is comparable to the size $l$ of the largest eddies. Plasmoid-mediated reconnection, which self-consistently occurs in the turbulent plasma, controls the physics of particle injection. Then, particles are further accelerated by stochastic scattering off turbulent fluctuations. The work done by parallel electric fields - naturally expected in reconnection layers - is responsible for most of the initial energy increase, and is proportional to the magnetization $\sigma$ of the system, while the subsequent energy gain, which dominates the overall energization of high-energy particles, is powered by the perpendicular electric fields of turbulent fluctuations. The two-stage acceleration process leaves an imprint in the particle pitch-angle distribution: low-energy particles are aligned with the field, while the highest energy particles move preferentially orthogonal to it. The energy diffusion coefficient of stochastic acceleration scales as $D_\gamma\sim 0.1\sigma(c/l)\gamma^2$, where $\gamma$ is the particle Lorentz factor. This results in fast acceleration timescales $t_{acc}\sim (3/\sigma)\,l/c$. Our findings have important implications for understanding the generation of nonthermal particles in high-energy astrophysical sources.