HOT ACCRETION DISKS WITH NONTHERMAL PAIRS - INJECTION OF LOW-ENERGY NONTHERMAL ELECTRONS

A model for X-ray and gamma-ray emitting regions around black holes is presented. We assume that the emission regions are optically thin acc retion disks where not only thermal processes but also nonthermal proc esses are important to disk structure and emission properties; we incl ude nonthermal electron injection, soft photon injection, e(+)e(-) pai r production, and subsequent pair cascades as well as radiation proces ses such as Compton scattering, bremsstrahlung, and pair annihilation. The emission spectrum from a disk is well described by a power law pr oduced by thermal and nonthermal Compton scatterings. The power-law in dex a, of X-rays is nearly independent of the mass accretion rate if l (soft)/l is constant, where alpha(x) is defined as F-nu proportional t o nu(alpha x), with F-nu being the energy spectrum, l(soft) is the com pactness of the injected soft photons, and l is the compactness of rel eased gravitational energy due to mass accretion. For example, alpha(x ) is about -0.8 and -2.0, when l(soft)/l = 0.2 and 2, respectively. Ou r model with a small l(soft)/l (<1) is promising to explain the low st ate observed from Galactic black hole candidates. Application of our m odel to Galactic black hole candidates, GX 339-4 and Cyg X-1, is shown as an example. Next, the effects of pair escape on disk structure are studied. Pair escape decreases pair density, and escaping pairs carry away thermal energy. However, these effects on the disk structure are minor. Finally, advective cooling/heating on disk structure as well a s the general relativistic effects on emission spectra are shown.