Optical properties of active galaxies with ultra-soft X-ray spectra

The optical properties of a sample of 53 active galactic nuclei (AGN) which exhibit an ultra-soft X-ray excess are examined. These AGN have been identified as part of the Ultra-Soft Survey (USS) which is selected from Einstein IPC sources. We present optical spectra of 27 of these AGN from which we measure continuum and line parameters, including fluxes, line FWHM and equivalent widths. Accurate B and R magnitudes are measured from CCD images. A UV spectrum of one USS AGN, EO132–411, has been obtained with IUE and is combined with the optical and X-ray data to produce a multiwavelength spectrum. We find a large proportion of narrow-line objects among the USS AGN: a comparison of the USS Hβ FWHM distribution with other X-ray selected samples confirms that the permitted lines of the USS AGN are biased to narrow widths. Two possible reasons for this are discussed; either we are looking at a face-on broad-line region (BLR), or the BLR lies further away from the central source than for other AGN. We compare optical line and continuum properties of the USS AGN with their separate hard and soft X-ray component luminosities and with the optical properties of other samples. We find that we are unable to distinguish the USS AGN from other X-ray selected objects on the basis of their optical luminosity. However, the strengths of the permitted lines are relatively low. The optical to X-ray ratio, αox, is calculated and we define a corresponding ratio for the soft component, αos. The effective αox for the USS AGN is high, 1.36 ± 0.04, and demonstrates that their mean hard X-ray luminosity is depressed relative to their mean optical luminosity. Optical Fe II emission is generally seen in those objects which have no significant hard X-ray emission, contradicting hard X-ray dependent models for their production, and favouring the Joly model which requires no ionizing continuum. All results are considered in the context of two models for the production of a strong soft X-ray flux, accretion discs and the cool clouds model of Guilbert & Rees. An association of a face-on BLR with an accretion disc in the USS AGN would strongly favour geometrically thick discs over thin.