Two-temperature accretion disks in pair equilibrium

We investigate two-temperature accretion disks with electron-positron pair production, taking account of the bremsstrahlung and Comptonization of soft photons produced by the cyclotron higher harmonics. The properties of the disks are qualitatively the same as those of disks in which bremsstrahlung is the only photon source. For an accretion rate higher than a critical value, Mcr, no steady solutions exist for a certain range of radial distance from a central black hole. The critical value increases only slightly with the input of soft photons; the increment is 45%, i.e., Mcr ∼ 0.43 MEdd, for the viscosity parameter α = 0.1, where MEdd ≡ LEdd/c2 = 4πGMBHmp/(σTc) with MBH being the mass of the central black hole. Furthermore, the disks are unstable against perturbations of the proton temperature. For α ∼ 0.1, the equipartition magnetic field, and a range of accretion rates, emission spectra obey the power law with a spectral index of -0.7 to -0.6, which coincides with the observed universal X-ray spectra of Seyfert galaxies. Brief comments on the model of the γ-ray flare of Cyg X-1 are also given. (author)