System-size convergence of nonthermal particle acceleration in relativistic plasma turbulence

TL;DR

This study uses large-scale particle-in-cell simulations to investigate how relativistic plasma turbulence accelerates particles, revealing that the resulting nonthermal energy spectra converge with increasing system size and may explain astrophysical observations.

Contribution

It demonstrates system-size convergence of nonthermal particle spectra in relativistic turbulence and links the power-law index to astrophysical phenomena.

Findings

Power-law energy distributions are produced by turbulence.

Convergence of spectra occurs at larger times with increasing system size.

The power-law index is approximately 3.0 for certain magnetization levels.

Abstract

We apply collisionless particle-in-cell simulations of relativistic pair plasmas to explore whether driven turbulence is a viable high-energy astrophysical particle accelerator. We characterize nonthermal particle distributions for varying system sizes up to $L/2\pi\rho_{e0} = 163$, where $L/2\pi$ is the driving scale and $\rho_{e0}$ is the initial characteristic Larmor radius. We show that turbulent particle acceleration produces power-law energy distributions that, when compared at a fixed number of large-scale dynamical times, slowly steepen with increasing system size. We demonstrate, however, that convergence is obtained by comparing the distributions at different times that increase with system size (approximately logarithmically). We suggest that the system-size dependence arises from the time required for particles to reach the highest accessible energies via Fermi acceleration. The converged power-law index of the energy distribution, $\alpha \approx 3.0$ for magnetization $\sigma = 3/8$, makes turbulence a possible explanation for nonthermal spectra observed in systems such as the Crab nebula.