UV TO X-RAY-SPECTRA OF RADIO-QUIET QUASARS - COMPARISON WITH ACCRETION DISK MODELS

We present UV to soft X-ray spectra of 31 radio-quiet quasars, compris ing 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 un derlying hard X-ray power law spectrum. Particularly for the steep X-r ay spectrum (alpha(energy) > 1.5), low redshift subsample (17 objects) the X-ray spectral power law index is strongly correlated with the op tical to X-ray broad-band spectral index alpha(ox), indicating that th e main contribution to the soft X-ray and optical emission is due to t he same emission component. We model the UV/soft X-ray spectra in term s of thermal emission from a geometrically thin alpha-accretion disk. The structure and radiation field of the disk is calculated self-consi stently and Compton scattering is treated by the Kompaneets equation. All relativistic effects on the disk structure and the emergent disk s pectrum 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 U V and soft X-ray spectra are well described by emission resulting from accretion rates in the range similar to 0.1 to similar to 0.3 times t he Eddington accretion rate. Low mass/low redshift objects are found t o accret at < 0.15 (M) over dot(Eddington) Correlations of the accreti on disk parameters with alpha(ox) are discussed.