Testing comptonization models using BeppoSAX observations of Seyfert 1 galaxies

We have used realistic Comptonization models to fit high-quality BeppoSAX d ata of six Seyfert galaxies. Our main effort was to adopt a Comptonization model taking into account the anisotropy of the soft photon field. The most important consequence is a reduction of the first scattering order, which produces a break (the so-called anisotropy break) in the outgoing spectra. Thus, anisotropic Comptonization models yield spectra with convex curvature s. The physical parameters of the hot corona (i.e., the temperature and opt ical depth) obtained by fitting this class of models to broadband X-ray spe ctra are substantially different from those derived by fitting the same dat a with the power law + cutoff model commonly used in the literature. In par ticular, our best fits with Comptonization models in slab geometry give a t emperature generally much larger and an optical depth much smaller than tho se derived from the power law + cutoff fits, using standard Comptonization formulae. The estimate of the reflection normalization is also larger with the slab corona model. For most objects of our sample, both models give Com pton parameter values larger than expected in a slab corona geometry, sugge sting a more "photon-starved" X-ray source configuration. Finally, the two models provide different trends and correlation between the physical parame ters: for instance, with the power law + cutoff fits, we obtain a correlati on between the reflection normalization and the corona temperature, whereas we find an anticorrelation between these parameters with the slab geometry . These differences have major consequences for the physical interpretation of the data. In the framework of reprocessing models, the cutoff power-law best-fit results suggest that the thermal corona is dominated by electron- positron pairs. On the contrary, the slab corona model is in better agreeme nt with a low pair density solution.