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

TL;DR

This study investigates the UV properties of soft X-ray selected Type I AGN using GALEX data, revealing new insights into the Big Blue Bump and its relation to X-ray spectra, with implications for accretion disk models.

Contribution

It provides the first systematic analysis of UV properties of faint, soft X-ray selected AGN and compares observed correlations with theoretical models of the soft X-ray excess.

Findings

BBB strength correlates with X-ray continuum shape.

BBB shape does not correlate with its relative strength.

Properties are relatively independent of black hole mass and Eddington ratio.

Abstract

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 Near-UV and Far-UV bands of the Galaxy Evolution Explorer (GALEX) satellite. Our sample is systematically fainter in the UV than galaxies studied in similar work by previous authors. We look for correlations between their UV and X-ray properties as well as correlations of these properties with either black hole mass or Eddington ratio. 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 alternative theories for the origin of the soft excess, but we are unable to distinguish between Comptonization of BBB photons by a hot plasma and absorption in relativistic winds as the most likely origins for the soft X-ray excess.