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 beta = p(gas)/p(mag). We find magnetically supported (low-beta), 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-beta solutions extend to high mass accretion rates (greater than or similar to alpha(2)(M)over dot(Edd)) and the electron temperature is moderately cool (T(e) similar to 10(8)-10(9.5) K). High luminosities (greater than or similar to 0.1L(Edd)) and moderately high energy cutoffs in the X-ray spectrum (similar to 50-200 keV) observed in the bright/hard state can be explained by the low-beta solutions.