Particle acceleration in relativistic turbulence: A theoretical appraisal

TL;DR

This paper investigates stochastic particle acceleration in relativistic MHD turbulence through simulations and analytical models, comparing different turbulence modes and extending understanding beyond traditional wave turbulence regimes.

Contribution

It combines numerical simulations with analytical estimates to analyze particle acceleration across various turbulence modes, including anisotropic effects and nonresonant mechanisms, providing practical formulas for astrophysical applications.

Findings

Fast, Alfvén, and slow modes contribute similarly to acceleration at high rigidities.

Resonance broadening enhances scattering rates for Alfvén and slow modes.

Analytical formulas are provided for nonresonant and small-scale electric field acceleration.

Abstract

We discuss the physics of stochastic particle acceleration in relativistic magnetohydrodynamic (MHD) turbulence, combining numerical simulations of test-particle acceleration in synthetic wave turbulence spectra with detailed analytical estimates. In particular, we study particle acceleration in wavelike isotropic fast mode turbulence, in Alfv\'en and slow Goldreich-Sridhar type wave turbulence (properly accounting for anisotropy effects), including resonance broadening due to wave decay and pitch-angle randomization. At high particle rigidities, the contributions of those three modes to acceleration are comparable to within an order of magnitude, as a combination of several effects (partial disappearance of transit-time damping for fast modes, increased scattering rate for Alfv\'en and slow modes due to resonance broadening). Additionally, we provide analytical arguments regarding acceleration beyond the regime of MHD wave turbulence, addressing the issue of nonresonant acceleration in a turbulence comprised of structures rather than waves, as well as the issue of acceleration in small-scale parallel electric fields. Finally, we compare our results to the existing literature and provide ready-to-use formulas for applications to high-energy astrophysical phenomenology.