Two-temperature accretion disks with electron-positron pair production

The authors investigate the effects of electron-positron pairs on a two-temperature accretion disk around a black hole, taking into account only bremsstrahlung. There are two branches of solution for a given accretion rate, i.e., the one with z < 1 and the other with z > 1, where z is the ratio of pair density to ion density. For an accretion rate lower than a critical value these two solutions are separated and continuous from an outer boundary to an inner boundary. On the other hand, for a higher accretion rate no steady-state solutions exist for a certain range of radial distance. The solution with z > 1 is shown to be unphysical for any accretion rate. When disks are in a state with z < 1, they are stable against secular perturbations but unstable against thermal perturbations, i.e., the fluctuations of ion temperature.