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 p resented. Together with the optical and ultraviolet spectra of this ga laxy, obtained within 8 months of the X-ray observations, these new da ta are used to determine the continuum energy distribution (CED) of Mr k 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 t otal energy in the broad emission lines. In particular, we find that t he observed CED can reproduce the observed line ratios and the luminos ity of the Lyalpha line, indicating that the clouds in the broad-line region see a continuum similar to the observed continuum.