Hot pair-dominated accretion disks

We use a powerful, recently discovered method to determine the structure and to analyze the properties of geometrically thin, steady, hot two-temperature accretion disks, where Comptonized bremsstrahlung is the dominant emission mechanism. The method exploits the fact that the disk solutions depend on two parameters only. This allows all possible local disk solutions to be displayed as a surface in a single figure. We emphasize that some of these local solutions may be unphysical or may not connect in a physically self-consistent way with solutions at different radii to form global disk solutions. Contrary to the results by others, we find local disk solutions at all accretion rates (having sub-Eddington luminosities), viscosity parameters, and radii. For a viscosity parameter, α < 1, we find narrow ranges of accretion rates or radii in between which there are three local disk solutions. At larger and smaller accretion rates or radii as well as for α > 1 there is only one local disk solution. The existence of a three-solution regime is due to folds in the solution surface caused by electron-positron pair production and radiation pressure becoming important. One global disk solution can be constructed for Ṁc²/L_Edd < α^0.2 when α < 1. For Ṁc²/L_Edd > α^0.2 and α < 1, disk gradients become infinite due to electron-positron pair production implying a breakdown of the geometrically thin disk assumptions. A slim disk approach is therefore needed in this case in order to determine the existence of global solutions to the steady disk equations. For 1 ≲ α ≲ 3, self-consistent pair-dominated global solutions are constructed. These disk solutions are viscously unstable at those radii where pairs and gas pressure dominate, but they are thermally and pair stable at all radii.