2021MNRAS.506.1128S

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 (>= 0.03 disc scale height H) show spatial diffusion coefficients of ∼30 and ∼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) ∝ p, and shear acceleration takes over when R_g >= 0.1H, characterized by D(p) ∝ p³. For particles with smaller R_g (<= 0.03H), their spatial diffusion coefficients roughly scale as ∼p^–1, and show evidence of D(p) ∝ p² 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.