Collisionless Black Hole Accretion

The accretion flows around the black holes in Sgr A*, M87, and other systems are strongly magnetized and collisionless. This in fact makes the usually employed general relativistic (GR) magnetohydrodynamic (MHD) method formally inapplicable. Thus, addressing the BH accretion problem in principle requires a fully kinetic approach. We study axisymmetric accretion of collisionless plasma around the black holes from first principles using GR particle-in-cell simulations (GRPIC). By doing so, we carry out a side-by-side comparison of global dynamics in GRMHD simulations and GRPIC for the same black hole accretion problem. Magnetic reconnection, which is believed to be responsible for particle acceleration and subsequent flares, is accurately captured in our kinetic approach. We directly examine the production of non-thermal particles due to magnetic reconnection. We discuss the implications of our results for event-horizon scale observations of Sgr A* and M87 by GRAVITY and the Event Horizon Telescope.