Soft excesses of hard X-ray selected Seyfert 1 galaxies studied with the ROSAT–PSPC

Abstract

The nature of the soft X-ray excess in Seyfert 1 galaxies is still an open issue. In order to derive some gen- eral properties we analysed the ROSAT PSPC archive ob- servations of the Seyfert galaxies in the Nandra & Pounds (1994) Ginga sample. The use of this sample ensures a re- liable estimate (under the assumption of lack of intrinsic spectral variability) of the slope of the underlying power law, which instead cannot be well measured using ROSAT data alone. We tested three dierent models for the soft excess: emission from accretion disk (described by a black body), absorption from ionized gas (\warm absorber, described by an absorption edge), and reflection from mildly ionized material (described by a step function). The main result is that no model gives a satisfactory t for all sources. Furthermore for two sources none of these models provides an acceptable t. In these cases a physically meaningful combination of two components (e.g. warm absorber plus black body) gives a good t. We conclude that several dierent eects concur to the soft excess phenomenon in Seyfert galaxies.