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Astron. Astrophys. 326, 885-896 (1997)

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UV to X-ray spectra of radio-quiet quasars. Comparison with accretion disk models

H. Brunner 1, C. Müller 2, P. Friedrich 1, T. Dörrer 2, R. Staubert 2 and H. Riffert 3

1 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany
2 Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuserstr. 64, D-72076 Tübingen, Germany
3 Institut für Astronomie und Astrophysik, Theoretische Astrophysik, Universität Tübingen, Auf der Morgenstelle, D-72076 Tübingen, Germany

Received 10 March 1997 / Accepted 11 June 1997


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 ([FORMULA]), 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 [FORMULA], 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 [FORMULA] -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 [FORMULA] 0.1 to [FORMULA] 0.3 times the Eddington accretion rate. Low mass/low redshift objects are found to accret at [FORMULA]. Correlations of the accretion disk parameters with [FORMULA] are discussed.

Key words: accretion disks – active galaxies – quasars – X-rays: galaxies – ultraviolet: galaxies

Send offprint requests to: H. Brunner

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998