SIMULTANEOUS OBSERVATIONS OF SEYFERT-1 GALAXIES WITH IUE, ROSAT AND GINGA

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 so ft X-ray spectral component in these sources. The unobservable far ult raviolet spectra are described in two ways, by using a power law fitte d to the (I 200-3200 angstrom) ultraviolet or by assuming that all sou rces have the same spectral slope in the extreme ultraviolet. Both des criptions indicate that the fraction of the luminosity of the ultravio let spectral component emitted at soft X-rays is very similar in the d ifferent sources. This can be expressed by similar maximum temperature s of the bump components (5 x 10(5) K). One of our spectral models, sh owing 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 luminos ity varies by a factor of 10(4) among the objects studied cannot be ex plained in the framework of current accretion disk or reprocessing mod els without fine tuning between the model parameters.