A statistical study of the relation between soft X-ray excess and accretion disk

Abstract

To study the origin of the soft X-ray excess, we compile a sample of 94 unobscured, radio-quiet QSOs and Seyfert galaxies with available data from GALEX and ROSAT. We find that 50 sources show strong soft X-ray excess and the other 44 show weak/no soft X-ray excess. Systematic analyses of the data indicate that the difference in soft X-rays is mainly but not only resulting from different accretion rates (in units of Eddington rate). The statistical study of the sources with soft X-ray excess shows that the strength of soft X-ray excess weakly and positively correlates with the Eddington ratio and increases with the increase of the strength of UV radiations relative to the X-rays. Provided that the UV emissions are from the thin disk, the correlations imply that the origin of soft X-ray excess is associated with the thin disk, either by means of Comptonization of the disk photons or in some other ways.