1994A&A...285..119W

Simultaneous observations of 8 Seyfert 1 type active galactic nuclei (Fairall-9, Mrk 590, NGC 4051, 3C 273, NGC 5548, Mrk 841, Q1821+643 and 3C 390.3) obtained with IUE, ROSAT and, for 5 targets, with Ginga are presented and analysed with the aim to describe the ultraviolet to soft X-ray spectral component in these sources. The unobservable far ultraviolet spectra are described in two ways, by using a power law fitted to the (1200-3200 A) ultraviolet or by assuming that all sources have the same spectral slope in the extreme ultraviolet. Both descriptions indicate that the fraction of the luminosity of the ultraviolet spectral component emitted at soft X-rays is very similar in the different sources. This can be expressed by similar maximum temperatures of the bump components (5.10⁵ K). One of our spectral models, showing a nearly constant spectral shape but a variable strength of the bump, is able to reproduce most of the observed spectra. Such similar ratios between the soft X-ray and ultraviolet fluxes while the luminosity varies by a factor of 10⁴ among the objects studied cannot be explained in the framework of current accretion disk or reprocessing models without fine tuning between the model parameters.