Particle diffusion and acceleration in magnetorotational instability turbulence

Hot accretion flows contain collisionless plasmas that are believed to be capable of accelerating particles to very high energies, as a result of turbulence generated by the magnetorotational instability (MRI). We conduct unstratified shearing-box simulations of the MRI turbulence in ideal magnetohydrodynamic (MHD), and inject energetic relativistic test particles in simulation snapshots to conduct a detailed investigation on particle diffusion and stochastic acceleration. We consider different amount of net vertical magnetic flux, with sufficiently high resolution to resolve the gyro-radii (R-g) of most particles. Particles with large R-g (greater than or similar to 0.03 disc scale height H) show spatial diffusion coefficients of similar to 30 and similar to 5 times Bohm values in the azimuthal and poloidal directions, respectively. We further measure particle momentum diffusion coefficient D(p) by applying the Fokker-Planck equation, finding that contribution from turbulent fluctuations scales as D(p) proportional to p, and shear acceleration takes over when R-g greater than or similar to 0.1H, characterized by D(p) proportional to p(3). For particles with smaller R-g (less than or similar to 0.03H), their spatial diffusion coefficients roughly scale as similar to p(-1), and show evidence of D(p) proportional to p(2) scaling in momentum diffusion but with large uncertainties. We find that multiple effects contribute to stochastic acceleration/deceleration, and the process is likely affected by intermittency in the MRI turbulence. We also discuss the potential of accelerating PeV cosmic rays in hot accretion flows around supermassive black holes.

Automated summary

This study investigates the impact of MRI turbulence in hot accretion flows on particle diffusion and stochastic acceleration, revealing patterns in spatial diffusion coefficients and momentum diffusion coefficients for particles with different gyro-radii. The results suggest that multiple factors contribute to stochastic acceleration/deceleration, influenced by intermittency in the MRI turbulence. The study also discusses the potential for accelerating PeV cosmic rays in hot accretion flows around supermassive black holes.