Accretion-disk modeling of the UV spectrum of quasars

The thin accretion disk spectrum, including the effect of electron scattering, is calculated for a grid of the accretion parameters. It is shown that the luminosity and spectral slope in the UV and optical bands are uniquely determined by these parameters and depend only weakly on the viscosity parameter. When the model calculations are compared with observations of a sample of QSOs and Seyfert 1 nuclei, the black hole mass is in the range 10 to the 8th to 10 to the 9.5 solar for the QSOs and 10 to the 7.5 to 10 to the 8.5 solar for the Seyferts and for low-luminosity objects. The correlation between the spectral slope and luminosity of QSOs in the UV can be explained by evolution along curves of constant black hole mass and decreasing accretion rate. The relation between the spectrum and the accretion parameters can be used to constrain the cosmological evolution of the objects and imposes severe constraints on the pure luminosity evolution interpretation function for QSOs and Seyferts.