Radiation properties of accreting gas onto compact objects

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1. We investigated the hydrostatic structure of atmospheric gas over an accretion disk, irradiated by UV/X-ray photons from a central source. We calculated radiation transfer coupling with balances between photoionization and recombination and between heating and cooling. It is found that the gas is thermally unstable at around the temperature of 100 eV. Also is found that the region could be subject to convective instability under gravity of the compact object, depending on the electron conduction.2. The radiation property of the gas is realized by strong photoionization; The gas becomes recombining and the emission thereof is dominated by narrow recombination continua and cascade lines due to electron capture into excited states. The similar condition is expected for afterglow of γ-ray bursts. We show the observed iron spectrum is explained by recombining radiation due to photoionization or rarefaction of the associated blast wave.3. Our model predicts that He II ionization front becomes optically thick with increasing the density of accreting gas. In side the front even heavy elements are fully ionized, while ionizing photons decreases rapidly across the front outside. For an X-ray binary, we find that iron of low ionic states survives even in the vicinity of the neutron star. This is evidence for formation of the optically thick front, which works to shield the radiation from the neutron star inside.4. In the study of accreting gas, we developed a radiation code for nonequilibrium ionization and non-Maxwellian distribution as well as non-LTE. We apply our scheme to elucidate the origin of the Galactic ridge emission; its energy source and emission mechanism were unresolved. We investigated the distribution function of electrons accelerated in the interstellar medium and calculated the radiation thereby. Our model can explain the observed characteristics successfully.All the results have been published.