Particle Diffusion and Acceleration in Magnetorotational Instability Turbulence

Abstract

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 to achieve different disk magnetizations levels at saturated states, with sufficiently high resolution to resolve the gyro-radii ($R_g$) of most particles. Particles with large $R_g$ ($\gtrsim 0.03$ disk scale height $H$) show spatial diffusion coefficients of $\sim 30$ and $\sim 5$ 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 to particle momentum evolution. For these particles, contribution from turbulent fluctuations scales as $D(p)\propto p$, and shear acceleration takes over when $R_g\gtrsim 0.1H$, characterized by $D(p)\propto p^3$. For particles with smaller $R_g$ ($\lesssim 0.03H$), their spatial diffusion coefficients roughly scale as $\sim p^{-1}$, and show evidence of $D(p)\propto p^2$ scaling in momentum diffusion but with large uncertainties. We find that multiple effects contribute to stochastic acceleration/deceleration, and the process is also 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.

22 pages, 18 figures, accepted for publication in MNRAS

References in corpus (23)

• First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring
• Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?
• Athena: A New Code for Astrophysical MHD
• Particle acceleration in relativistic collisionless shocks: Fermi process at last?
• Formation of Hard Power-laws in the Energetic Particle Spectra Resulting from Relativistic Magnetic Reconnection
• The Fermi Bubbles: Giant, Multi-Billion-Year-Old Reservoirs of Galactic Center Cosmic Rays
• Numerical Simulation of Hot Accretion Flows (III): Revisiting wind properties using trajectory approach
• Cosmic Ray Propagation: Nonlinear Diffusion Parallel and Perpendicular to Mean Magnetic Field
• On the Momentum Diffusion of Radiating Ultrarelativistic Electrons in a Turbulent Magnetic Field
• Angular Momentum Transport in Accretion Disks: Scaling Laws in MRI-driven Turbulence
• Kinetic turbulence in relativistic plasma: from thermal bath to non-thermal continuum
• Simulations of Electron Magnetohydrodynamic Turbulence
• Magnetorotational Turbulence and Dynamo in a Collisionless Plasma
• Hybrid-Kinetic Simulations of Ion Heating in Alfvénic Turbulence
• Electron Heating by the Ion Cyclotron Instability in Collisionless Accretion Flows. I. Compression-Driven Instabilities and the Electron Heating Mechanism
• The Radiative Efficiency and Spectra of Slowly Accreting Black Holes from Two-Temperature GRRMHD Simulations
• A microscopic analysis of shear acceleration
• Angular Momentum Transport and Particle Acceleration during Magnetorotational Instability in a Kinetic Accretion Disk
• Particle Acceleration in Mildly Relativistic Shearing Flows: the Interplay of Systematic and Stochastic Effects, and the Origin of the Extended High-energy Emission in AGN Jets
• Electron-scale shear instabilities: magnetic field generation and particle acceleration in astrophysical jets
• Temperature Fluctuations driven by Magnetorotational Instability in Protoplanetary Disks
• Shear Acceleration in Expanding Flows
• Universal small-scale structure in turbulence driven by magnetorotational instability

Cited by in corpus (1)

• Calliope: Pseudospectral shearing magnetohydrodynamics code with a pencil decomposition