THE EXTREME-ULTRAVIOLET EMISSION OF THE SEYFERT-GALAXIES MARKARIAN-279, MARKARIAN-478, AND TON-S180

We present EUV light curves and spectra of three Seyfert galaxies. The data were obtained from EUVE observations of the galaxies Mrk 279, Mr k 478, and Ton S180. The variability in the light curves provides an u pper limit to the size of the EUV-emitting region which excludes the b road-line region and diffuse intercloud regions as the source of this emission. The EUV spectra are all characterized by discrete line featu res; in the case of Mrk 478, we find the lines vary on timescales of l ess than 1 day. These results allow us to rule out various models prop osed for Seyfert galaxies. Comparing these spectra in their rest frame s, we find that several of these features appear at the same wavelengt hs. A composite EUV spectrum is produced from these redshift-corrected spectra. The most striking feature in the composite spectrum is an em ission feature at similar to 79 Angstrom, which is also observed in th e spectra of each of the individual galaxies. It is difficult to expla in this feature by emission from a photoionized or collision-driven pl asma in thermal equilibrium. We suggest this line feature may be the f luorescent transition of neutral Fe related to the 6.4 keV K alpha emi ssion that has frequently been observed in the X-ray spectra of Seyfer t galaxies. If this interpretation is correct, this result imposes str ong constraints on the distribution of warm material in these galaxies .