Thermal equilibria of optically thin, magnetically supported, two-temperature, black hole accretion disks

We obtained thermal equilibrium solutions for optically thin, two-temperature black hole accretion disks incorporating magnetic fields. The main objective of this study is to explain the bright/hard state observed during the bright/slow transition of galactic black hole candidates. We assume that the energy transfer from ions to electrons occurs via Coulomb collisions. Bremsstrahlung, synchrotron, and inverse Compton scattering are considered as the radiative cooling processes. In order to complete the set of basic equations, we specify the magnetic flux advection rate instead of β = p _gas/p _mag. We find magnetically supported (low-β), thermally stable solutions. In these solutions, the total amount of the heating via the dissipation of turbulent magnetic fields goes into electrons and balances the radiative cooling. The low-β solutions extend to high mass accretion rates (≳ α²M_Edd) and the electron temperature is moderately cool (T _e ∼ 10⁸-10 ^9.5 K). High luminosities (≳0.1L _Edd) and moderately high energy cutoffs in the X-ray spectrum (50-200 keV) observed in the bright/hard state can be explained by the low-β solutions.