A&A Supplement series, Vol. 126, December II 1997, 1-10
Received January 8; accepted April 17, 1997
L. Piro
- G. Matt
- R. Ricci,
Send offprint request: Luigi Piro: piro@alpha1.ias.fra.cnr.it
Istituto di Astrofisica Spaziale - C.N.R., Via E. Fermi 21, I-00044
Frascati(RM), Italy
Dipartimento di Fisica, Università degli Studi "Roma Tre'', Via della Vasca
Navale 84, I-00146 Roma, Italy
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 Nandra & Pounds (1994) 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.
keywords: galaxies: Seyfert; X-rays: galaxies