1996A&A...309...81W

ROSAT spectra of 86 Active Galactic Nuclei (AGN) with a broad range of optical FeII strengths have been analyzed. The results of the spectral analysis have been combined with optical and radio data as well as with optical emission line properties collected from the literature to understand the origin of the strong FeII emission and the nature of the soft X-ray excess. The main results of this paper are: (1) The intensity ratio I(FeII)/I(Hβ), the power law photon index of the soft X-ray spectra and the line width of Hβ are strongly correlated with each other. Our analysis suggests that these correlations could be caused by a strong link to a common parameter. (2) The equivalent width of the optical FeII emission is significantly correlated with the soft X-ray photon index and the X-ray luminosity at 2keV. The equivalent width of Hβ is significantly anti-correlated with the soft X-ray photon index and the optical to X-ray spectral index. The correlations between the equivalent width of Hβ or FeII and the soft X-ray photon index probably have a different origin. (3) About 10% of the objects show strong signatures of ionized absorption. In one object, PG1126-041, strong ionized UV absorption lines are present in low resolution IUE spectra. Most objects with ionized absorption have strong FeII emission and all have a steep optical to X-ray spectral slope. (4) The correlation between the Hβ line width and the X-ray luminosity is consistent with the assumption of a gravitationally bound Broad Line Region (BLR). We can rule out a direct relation between the X-ray emission of extreme FeII AGN and starburst activity because of the high soft X-ray luminosities and the steepness of the soft X-ray spectra. In about at least one third of the cases, the soft X-ray flux in the ROSAT band cannot provide sufficient heating for the FeII emission region. This suggests that current photoionization models for the FeII emission are problematic. The most likely interpretation for the correlation between the soft X-ray photon index {GAMMA}_x and the Hβ line width is that both parameters are linked to the accretion rate and the mass of the central black hole.