EUV Spectroscopy of X-ray Photoionized Plasmas

A soft X-ray excess near 1 keV has been revealed in the X-ray spectra of many Seyfert galaxies, possibly attributable to an Fe L-shell emission-line complex. If this is the case, an EUV counterpart, produced by 2p-2s transitions in Fe XVIII-XXIV, must accompany the soft X-ray complex. We have proposed to search for the EUV line emission in the Seyfert galaxy Mrk 478, the only Seyfert detected in the ROSAT WFC survey. Line emission is likely to be influenced by the presence of the hard X-ray continuum which characterizes Seyferts. We have developed a set of spectral models which can be used to discriminate between photoionization-dominated and collision-dominated line emission, as well as an array of quantitative plasma diagnostics. Detection of EUV line emission represents a unique opportunity to derive several astrophysical parameters in the accretion flow surrounding the central black hole and to establish the nature of the soft X-ray excess in AGN.