We test accurate models of Comptonization spectra over the high quality data of the Beppo SAX long look at NGC 5548 .
The data are well represented by a plane parallel corona with an inclination angle of 30 ^ { \circ } , a soft photon temperature of 5 eV and a hot plasma temperature and optical depth of kT _ { e } \simeq 360 keV and \tau \simeq 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 to 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 ” curvature which moves the fitted high energy cut-off to higher energies .

KEYWORDS : Radiation mechanisms : thermal ; Methods : data analysis ; Beppo SAX ; Galaxies : Seyfert ; Galaxies : individual : NGC 5548