ROSAT XRT-PSPC OBSERVATIONS AND THE IONIZING CONTINUUM OF THE SEYFERT-1 GALAXY MRK-478

The ROSA T/XRT-PSPC observations of the Seyfert 1 galaxy Mrk 478 are presented. Together with the optical and ultraviolet spectra of this galaxy, obtained within 8 months of the X-ray observations, these new data are used to determine the continuum energy distribution (CED) of Mrk 478, from 1 mum to about 2.0 keV with a small break between 11.3 eV and 0.1 keV. The ultraviolet/soft X-ray CED is similar to the spectrum of a thin accretion disc for both a rotating and a non-rotating black hole, although both models underestimate the flux at energies higher than 0.4 keV These observations establish that the covering factor of the clouds in the broad-line region in this Seyfert 1 galaxy is about 0.15 and that the energy of the ionizing continuum is similar to the total energy in the broad emission lines. In particular, we find that the observed CED can reproduce the observed line ratios and the luminosity of the Lyalpha line, indicating that the clouds in the broad-line region see a continuum similar to the observed continuum.