First-principles demonstration of diffusive particle acceleration in kinetic simulations of relativistic plasma turbulence

Kai Wong, Vladimir Zhdankin, Dmitri A. Uzdensky, Gregory R. Werner, Mitchell C. Begelman

Published 2019-01-11Version 1

Nonthermal relativistic plasmas are ubiquitous in astrophysical systems like pulsar wind nebulae and active galactic nuclei, as inferred from their emission spectra. The underlying nonthermal particle acceleration (NTPA) processes have traditionally been modeled with a Fokker-Planck (FP) equation in momentum space. In this paper, we directly test the FP framework in ab-initio kinetic simulations of driven magnetized turbulence in relativistic pair plasma. By statistically analyzing the motion of tracked particles, we demonstrate the diffusive nature of NTPA and measure the FP energy diffusion ($D$) and advection ($A$) coefficients as functions of particle energy $\gamma m_e c^2$. We find that $D(\gamma)$ is proportional to $\gamma^2$ in the high-energy nonthermal tail, in line with 2nd-order Fermi acceleration theory, but has a much shallower scaling of about $\gamma^{2/3}$ at lower energies. We also find that $A$ tends to pull particles towards the peak of the distribution. This study provides strong support for the FP picture of turbulent NTPA, thereby enhancing our understanding of space, solar, and astrophysical plasmas.