HOT ACCRETION DISKS WITH NONTHERMAL PAIRS

The structure of hot accretion disks in active galactic nuclei (AGNs) is investigated, including soft photon injection, nonthermal relativistic electron injection, and pair cascades. A part of gravitational energy is used to accelerate nonthermal electrons, which produce gamma-rays by Compton scattering of soft photons. Those gamma-rays then create e(+-) pairs by photon-photon collisions. Pairs are thermalized before annihilation. Thermalized pairs contribute to thermal Compton scattering together with ionized electrons of accreting matter. The structure of a disk in the (surface density, accretion rate) plane is qualitatively the same as that of pure thermal disks, where no injection of nonthermal electrons occurs. There are three branches of solutions for a given surface density: the lower branch with a very low pair density and dominated by gas pressure, the middle branch dominated by pairs and gas pressure, and the higher branch dominated by pairs and radiation pressure. Critical rates related to pair production and radiation pressure, known to exist for pure thermal disks, are not found for the parameter sets used. The emission spectrum is dominated mainly by thermal and nonthermal Compton scattering of soft photons, which produces the power-law X-rays observed from ACNs. When the luminosity is high, e.g., more than 30% of Eddington luminosity if 30% of gravitational energy is directed to nonthermal injection, the feature of pair annihilation radiation appears around 511 keV. Our results show that sources of nonthermal radiation can exist self-consistently in a hot accretion disk near the central black hole. The validity of this model might be tested by future observations of pair annihilation lines.