A self-consistent test of Comptonization models using a long BeppoSAX observation of NGC 5548

We test accurate models of Comptonization spectra using the high quality data of the BepposSAX long look at NGC 5548. The data are well represented by a plane parallel corona with an inclination angle of 30°, a soft photon temperature of 5 eV and a hot plasma temperature and optical depth of kT e ≃360 keV and τ≃0.1, respectively. If energy balance applies, such values suggest that a more “photon-starved” geometry (e.g. a hemispheric region) is necessary. The spectral softening detected during a flare appears to be associated with a decrease of the heating-to-cooling ratio, indicating a geometric and/or energetic modification of the disk plus corona system. The hot plasma temperature derived with the models above is significantly higher than that obtained fitting the same data with a power law plus high energy cut off model for the continuum. This is due to the fact that in anisotropic geometries Comptonization spectra show “intrinsic” curvatures which move the fitted high energy cut-off to higher energies.