GALEX Measurements of the Big Blue Bump in Soft X-ray Selected AGN

We study the UV properties of Type I AGN from the ROSAT All-Sky Survey that have been selected to show unusually soft X-ray continua. We examine a sample of 54 Seyfert 1 galaxies with detections in both the GALEX Near-UV and Far-UV bands and look for correlations between their UV and X-ray properties. We also search for correlations of these properties with either black hole mass or Eddington ratio. Our sample is systematically fainter in the UV than galaxies studied in similar work by previous authors, revealing a population of soft excess galaxies with weaker UV fluxes than previously reported. The shape of the Big Blue Bump (BBB) in the GALEX regime does not appear to correlate with its strength relative to the power law continuum, which conflicts with results reported by previous authors. The strength of the BBB is correlated with the shape of the X-ray continuum, in agreement with previous work, but the slope of the correlation is different than previously reported. The properties of the accretion disks of Type I AGN in the GALEX regime are relatively independent of black hole mass and Eddington ratio. We compare our measurements to the predictions of several theories for the origin of the soft excess and find that models invoking Comptonization of BBB photons by a hot plasma agree best with our results. Subject headings: galaxies: Seyfert, ultraviolet: general, X-ray: general