Testing comptonization models using BeppoSAX observations of Seyfert 1 galaxies

We have used realistic Comptonization models to fit high-quality BeppoSAX data 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 curvatures. The physical parameters of the hot corona (i.e., the temperature and optical depth) obtained by fitting this class of models to broadband X-ray spectra are substantially different from those derived by fitting the same data with the power law + cutoff model commonly used in the literature. In particular, our best fits with Comptonization models in slab geometry give a temperature generally much larger and an optical depth much smaller than those 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 Compton parameter values larger than expected in a slab corona geometry, suggesting a more "photon-starved" X-ray source configuration. Finally, the two models provide different trends and correlation between the physical parameters: for instance, with the power law + cutoff fits, we obtain a correlation 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 agreement with a low pair density solution.