Global Compton Heating and Cooling in Hot Accretion Flows

The hot accretion flow (such as advection-dominated accretion flow) is usually optically thin in the radial direction, therefore the photons produced at one radius can travel for a long distance without being absorbed and heat or cool electrons at other radii via Compton scattering. This effect has been ignored in most previous works on hot accretion flows and is the focus of this paper. If the mass accretion rate is described by Ṁ = Ṁ0(r/rout) with rout = 10rs and rs = 2GM/c2, we find that when Ṁ0 > 0.1LEdd/c, the rates of Compton heating (at r & 10rs) or cooling (at r . 10rs) are larger than the local heating rate of electrons; therefore the effect is important. We can obtain the self-consistent steady solution with this effect included only if the accretion rate Ṁ0 . LEdd/c which corresponds to L . 0.01LEdd. Above this accretion rate the equilibrium temperature of electrons at rout = 10rs is higher than the virial temperature, as a result of strong Compton heating, so the accretion is suppressed. We argue that in this case the activity of the black hole will “oscillate” between an active and an inactive phases, with the oscillation timescale being the radiative timescale of the gas at rout. Subject headings: accretion, accretion disks — black hole physics — galaxies: active — quasars: general — X-rays: general