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, Mrk 478, and Ton S180. The variability in the light curves provides an upper limit to the size of the EUV-emitting region which excludes the broad-line region and diffuse intercloud regions as the source of this emission. The EUV spectra are all characterized by discrete line features; in the case of Mrk 478, we find the lines vary on timescales of less than 1 day. These results allow us to rule out various models proposed for Seyfert galaxies. Comparing these spectra in their rest frames, we find that several of these features appear at the same wavelengths. A composite EUV spectrum is produced from these redshift-corrected spectra. The most striking feature in the composite spectrum is an emission feature at ~79 Å, which is also observed in the spectra of each of the individual galaxies. It is difficult to explain this feature by emission from a photoionized or collision-driven plasma in thermal equilibrium. We suggest this line feature may be the fluorescent transition of neutral Fe related to the 6.4 keV Kα emission that has frequently been observed in the X-ray spectra of Seyfert galaxies. If this interpretation is correct, this result imposes strong constraints on the distribution of warm material in these galaxies.