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

The nature of the soft X-ray excess in Seyfert 1 galaxies is still an open issue. In order to derive some general properties we analysed the ROSAT PSPC archive observations of the Seyfert galaxies in the \cite[Nandra & Pounds (1994)]{Nan94} Ginga sample. The use of this sample ensures a reliable 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 different 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 fit for all sources. Furthermore for two sources none of these models provides an acceptable fit. In these cases a physically meaningful combination of two components (e.g. warm absorber plus black body) gives a good fit. We conclude that several different effects concur to the soft excess phenomenon in Seyfert galaxies.