X-RAY PHOTOIONIZED ACCRETION DISKS - UV AND X-RAY CONTINUUM SPECTRA AND POLARIZATION

We calculate the UV and X-ray continuum spectrum and polarization from an accretion disc centrally illuminated by X-rays. The calculation of the ionization structure of the disc is performed by using the numerical code described by Ross & Fabian, while the inclination effects and the polarization are calculated by solving the transfer equation for polarized radiation. The general relativistic effects in the Schwarzschild metric are also taken into account. The spectra we obtain can explain the X-ray observation for some active galactic nuclei and several Galactic black hole candidates, for which both the iron-line intensity and the shape of the reflection continuum appear to be inconsistent with reprocessing from neutral matter. We find that the degree and angle of polarization vary strongly with the energy; this fact should be testable by future observations. We also suggest, on the basis of earlier works by Loskutov & Sobolev, that the absorption opacity solution of the polarization problem for thin discs can reconcile theory with observations, since even a small absorption opacity can strongly reduce the polarization degree to very low, if observable, values.