Observational evidence for thin AGN disks

Observational evidence for thin AGN disks include their spectrum and spectral features, polarization, inclination and X-ray line and continuum reflection features.These are discussed, in a critical way, in an attempt to find the ones that are the least model dependent. The optical and ultraviolet continuum of AGNs can be explainedin various different ways, one of which is thermal emission by geometrically thin accretion disks. The continuum emission of such disks is not yet understood well enough to clearly distinguish this from other continuum emitting processes. This is also the case for several spectral features, including absorption and emission edges, proposed to be clear disk-emission signatures. The sign and the strength of such edges are model dependent and relativistic broadening and shifting makes them hard to detect. Polarization observations at ultraviolet wavelengths, are more promising. In particular, the presence or absence of the predicted Lyman edge polarization feature may be used as a decisive test for thin, bare AGN disks. Other good model independent tests are several inclination related line and continuum correlations, in big AGN samples. The illumination of thin AGN disks by a central X-ray source gives rise to several detectable X-ray features. Present modeling of these effects lacks a complete treatment of the temperature and ionization structure of the disk atmosphere. It is shown that the electron temperature near the surface of the disk can much exceed the disk equilibrium temperature, which causes deviations from LTE. Such effect must be incorporated into realistic disk models.