Black Hole Accretion in Low States: Electron Heating

TL;DR

This paper investigates electron heating mechanisms in low-state black hole accretion flows, emphasizing the importance of collision-less turbulence heating over Coulomb collisions, and determines the critical accretion rate for two-temperature solutions.

Contribution

It provides observational evidence and theoretical analysis showing collision-less turbulence heating significantly influences electron temperatures in low accretion states.

Findings

Efficient electron heating by turbulence observed in low accretion states.

Critical accretion rate for two-temperature flow depends weakly on collision-less heating.

Collision-less turbulence heating is essential for accurate modeling of black hole accretion flows.

Abstract

Plasmas in an accretion flow are 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 observational evidence of efficient electron heating by turbulence or collective plasma effects in low accretion states, when Coulomb collisions are not efficient enough to establish a thermal equilibrium between electrons and ions at small radii. However, charged particles reach a thermal equilibrium with their kind much faster than with others through Coulomb collisions, a two-temperature accretion flow is expected. 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 solution may exist, is determined by the cooling processes and the collisional energy exchanges 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. Collision-less heating of electrons by MHD turbulence can no longer be ignored in quantitative investigations of these systems.