2001A&A...375..328N

The UV/X-ray irradiated gas around a compact object was studied by solving the hydrostatic balance coupling with the ionization and thermal balances to obtain the structure over the accretion disk. The gas is ionized and heated by the strong radiation from the compact source, and becomes a recombining plasma; the kinetic energy of the particles is much lower than the energy of the ionization state. The characteristic thermal properties significantly affect the cooling rate, and consequently the structure of the gas. Under gravitation due to the compact object, the structure of the gas is classified into three distinct regimes: (I) a dynamically stable region of high temperature, (II) a stable region of low temperature, and (III) an unstable two-phase region of intermediate temperature. We found that the stability condition dT/dΞ>0 does not simply apply to the photoionized gas under gravity, where T is the gas temperature and Ξ is the ratio of the radiation pressure to the gas pressure. Regions I and II are located in that order from the central source toward the outer disk, and region III forms at the interface between the two stable regions. The two phases of high and low temperatures in region III could be smeared marginally by electron conduction, but the dynamically unstable part likely remains at the temperature of kT∼100eV. This characteristic temperature appears purely due to atomic processes and is independent of the source parameters such as the mass or the luminosity of the system. This temperature is of critical importance to the ionization state and radiation mechanisms in the gas. The line-like narrow recombination continua and recombination-cascade lines dominate in the region below ∼100eV, while the featureless free-free continuum dominates the radiation spectrum in the region of higher temperatures. These properties are widely true of accretion-powered sources, from stellar-mass systems like X-ray binaries to massive systems like active galactic nuclei, and should be taken into account for the gas dynamics of the accretion flows.