Black Hole Accretion in Low States: Electron Heating

Gas in an accretion flow is heated by MHD turbulence generated through the magneto-rotational instability. The viscous stress driving the accretion is intimately connected to the microscopic processes of turbulence dissipation. We show that, in a few well-observed black hole accretion systems, there is compelling evidence of efficient electron heating by turbulence or collective plasma effects in the two-temperature accretion states. We consider a Keplerian accretion flow with a constant mass accretion rate in the pseudo-Newtonian gravitational potential and take into account the bremsstrahlung, synchrotron, and inverse Comptonization cooling processes. The critical mass accretion rate, below which the two temperature solutions exist, is determined by the cooling processes and the collisional energy exchange between electrons and ions and has very weak dependence on the collision-less heating of electrons by turbulence, which becomes more important at lower accretion rates. Quantitative investigations of the relevant plasma processes through detailed modeling and numerical simulations are warranted. Subject headings: acceleration of particles — accretion, accretion disks — black hole physics — plasmas — radiation mechanisms: thermal— turbulence