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We reexamine the hypothesis that the optical/UV/soft X-ray continuum ofActive Galactic Nuclei is thermal emission from an accretion disk. Previousstudies have shown that fitting the spectra with the standard, optically thickand geometrically thin accretion disk models often led to luminosities whichcontradict the basic assumptions adopted in the standard model. There is noknown reason why the accretion rates in AGN should not be larger than the thindisk limit. In fact, more general, slim accretion disk models areself-consistent even for moderately super-Eddington luminosities. We calculatehere spectra from a set of thin and slim, optically thick accretion disks. Wediscuss the differences between the thin and slim disk models, stressing theimplications of these differences for the interpretation of the observedproperties of AGN. We found that the spectra can be fitted not only by modelswith a high mass and a low accretion rate (as in the case of thin disk fitting)but also by models with a low mass and a high accretion rate. In the first casefitting the observed spectra in various redshift categories gives black holemasses around 10^9 solar masses for a wide range of redshifts, and foraccretion rates ranging from 0.4 to 8 solar masses/year. In the second case theaccretion rate is around 10^2 solar masses/year for all AGN and the mass rangesfrom 3*10^6 to 10^8 solar masses. Unlike the disks with a low accretion rate,the spectra of the high-accretion-rate disks extend into the soft X-rays. Acomparison with observations shows that such disks could produce the soft X-rayexcesses claimed in some AGNs. We show also that the sequence of our modelswith fixed mass and different accretion rates can explain the time evolution ofthe observed spectra in Fairall 9.Comment: LaTeX file, 21 pages, 21 figures (on request from ewa@sissa.it)  accepted by Astrophysical Journa

The Observational Appearance of Slim Accretion Disks