New bright soft X-ray selected ROSAT AGN I. Infrared-to-X-ray spectral energy distributions

We present results of an infrared-to-X-ray study of 76 bright soft X-ray selected Seyfert galaxies discovered in the ROSAT All-Sky Survey. These objects are characterized by steep X-ray spectra in the 0.2-2.0 keV bandpass with power law energy spectral indices in the range of 1.3 to 8 and a lack of internal absorption by neutral hydrogen. Our sample selection based on hardness ratio yields a mean slope of alpha(X) = 2.1 +/- 0.1 (F-nu proportional to nu(-alpha)), steeper than in any other known AGN population. At optical wavelengths, the soft AGN have significantly bluer spectra than a comparison sample of AGN with a canonical, harder X-ray spectrum, whereas the slope between 5500 Angstrom and 1 keV is the same. This is consistent with a more pronounced Big Blue Bump emission component in the soft X-ray selected AGN. The blueness of the optical spectra increases with the softness of the X-ray spectra and with the luminosity, saturating at an approximate F-nu proportional to nu(+0.3) spectrum. Such properties are expected if most of the Big Blue Bump emission originates in a (comptonized) accretion disk and M/M is higher than in AGN with a hard X-ray spectrum.