Collisionless Accretion onto Black Holes: Dynamics and Flares

We study the accretion of collisionless plasma onto a rotating black hole from first principles using axisymmetric general-relativistic particle-in-cell simulations. We carry out a side-by-side comparison of these results to analogous general-relativistic magnetohydrodynamic simulations. Although there are many similarities in the overall flow dynamics, three key differences between the kinetic and fluid simulations are identified. Magnetic reconnection is more efficient, and rapidly accelerates a nonthermal particle population, in our kinetic approach. In addition, the plasma in the kinetic simulations develops significant departures from thermal equilibrium, including pressure anisotropy that excites kinetic-scale instabilities, and a large field-aligned heat flux near the horizon that approaches the free-streaming value. We discuss the implications of our results for modeling event-horizon scale observations of Sgr A* and M87 by GRAVITY and the Event Horizon Telescope.

Automated summary

We use axisymmetric general-relativistic particle-in-cell simulations to study the accretion of collisionless plasma onto a rotating black hole. Comparisons with analogous general-relativistic magnetohydrodynamic simulations reveal three key differences between the kinetic and fluid simulations. Our kinetic approach shows more efficient magnetic reconnection, which rapidly accelerates a nonthermal particle population. The plasma in the kinetic simulations also develops significant deviations from thermal equilibrium, including pressure anisotropy that excites kinetic-scale instabilities and a large field-aligned heat flux near the horizon.