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 α X = 2.1 ± 0.1 (F ν ∝ ν −α), 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 canon-ical, harder X-ray spectrum, whereas the slope between 5500Å 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 ν ∝ ν +0.3 spectrum. Such properties are expected if most of the Big Blue Bump emission originates in a (comp-tonized) accretion disk and ˙ M /M is higher than in AGN with a hard X-ray spectrum. ⋆ Based in part on observations at the European Southern Observatory La Silla (Chile) with the 2.2m telescope of the Max-Planck-Society during MPG and ESO time, and the ESO 1.5m telescope