  We reexamine the hypothesis that the optical/UV/soft X-ray continuum of
Active Galactic Nuclei is thermal emission from an accretion disk. Previous
studies have shown that fitting the spectra with the standard, optically thick
and geometrically thin accretion disk models often led to luminosities which
contradict the basic assumptions adopted in the standard model. There is no
known reason why the accretion rates in AGN should not be larger than the thin
disk limit. In fact, more general, slim accretion disk models are
self-consistent even for moderately super-Eddington luminosities. We calculate
here spectra from a set of thin and slim, optically thick accretion disks. We
discuss the differences between the thin and slim disk models, stressing the
implications of these differences for the interpretation of the observed
properties of AGN. We found that the spectra can be fitted not only by models
with 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 case
fitting the observed spectra in various redshift categories gives black hole
masses around 10^9 solar masses for a wide range of redshifts, and for
accretion rates ranging from 0.4 to 8 solar masses/year. In the second case the
accretion rate is around 10^2 solar masses/year for all AGN and the mass ranges
from 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. A
comparison with observations shows that such disks could produce the soft X-ray
excesses claimed in some AGNs. We show also that the sequence of our models
with fixed mass and different accretion rates can explain the time evolution of
the observed spectra in Fairall 9.
