  We present UV to soft X-ray spectra of 31 radio-quiet quasars, comprising
data from the IUE ULDA database and the ROSAT pointed observation phase. 90 %
of the sample members show a soft X-ray excess above an underlying hard X-ray
power law spectrum. Particularly for the steep X-ray spectrum (alpha_energy >
1.5), low redshift subsample (17 objects) the X-ray spectral power law index is
strongly correlated with the optical to X-ray broad-band spectral index
alpha_ox, indicating that the main contribution to the soft X-ray and optical
emission is due to the same emission component. We model the UV/soft X-ray
spectra in terms of thermal emission from a geometrically thin alpha-accretion
disk. The structure and radiation field of the disk is calculated
self-consistently and Compton scattering is treated by the Kompaneets equation.
All relativistic effects on the disk structure and the emergent disk spectrum
are included. Satisfactory spectral fits of the UV and soft X-ray continuum
spectra are achieved when additional non-thermal hard X-ray and IR power law
emission components are taken into account. The UV and soft X-ray spectra are
well described by emission resulting from accretion rates in the range ~0.1 to
~0.3 times the Eddington accretion rate. Low mass/low redshift objects are
found to accret at < 0.15 Mdot_Eddington. Correlations of the accretion disk
parameters with alpha_ox are discussed.
