PAIR-DENSITY TRANSITIONS IN ACCRETION DISK CORONAE

We investigate the thermal and e+e-(-)pair equilibrium structure of two-temperature disk coronae above a cool (T approximately 10(6) K) disk around a black hole of 10 M.. Soft photons are assumed to be amply supplied from the cool disk. We then find two pair thermal equilibrium points for a given proton column density: the low state with very small pair density and the high state dominated by pairs. Both states are thermally unstable, while for perturbations in pair density the high state is unstable and the low state is stable. Two possible scenarios are discussed for the fate of a two-temperature corona. When the proton optical depth, tau-p, is relatively small (e.g., less than 1) and the temperature of input soft photons is low (e.g., T(soft) less-than-or-similar-to 10(6) K), the corona will undergo a limit cycle between the high state and the low state on a time scale of milliseconds. As a consequence of Compton scattering of the soft photons, the emergent spectrum in the high state is rather flat with a big Wien bump at approximately 100 keV, whereas it is composed of a power-law component in the low state. When tau-p is relatively large (e.g., tau-p = 2) or T(soft) greater-than-or-similar-to 2 x 10(6) K, in contrast, the plasma will cool to form a one-temperature corona. The emergent spectrum is then almost a blackbody with energy approximately 1 keV. Some observational consequences are briefly discussed in connection with the high-low spectral transition in Cyg X-1.