Collisionless Black Hole Accretion 

The accretion flows around the black holes in Sgr A*, M87, and other systems are strongly magnetized and collisionless. This makes the usually employed general relativistic (GR) magnetohydrodynamic (MHD) method formally inapplicable. Modeling BH accretion thus in principle requires a fully kinetic approach. In this talk, I will show a study of axisymmetric accretion of collisionless plasma around black holes from first principles using GR particle-in-cell simulations (GRPIC). By doing so, I carry out a side-by-side comparison of global dynamics in GRMHD simulations and GRPIC for the same black hole accretion problem. The accreting plasma develops anisotropy in its distribution function with respect to the magnetic field direction. In addition to changes in dynamics and thermodynamics of the system, the temperature anisotropy can significantly change the properties of synchrotron emission. I will discuss these effects and their implications for near-horizon BH imaging such as that produced by the Event horizon telescope. I will also describe how the anisotropy of the plasma can affect future multi-frequency and photon ring observations.