We test accurate models of Comptonization spectra over the high quality data of the Beppo SAX long look at NGC 5548 , allowing for different geometries of the scattering region , different temperatures of the input soft photon field and different viewing angles .
We find that the Beppo SAX data are well represented by a plane parallel or hemispherical corona viewed at an inclination angle of 30 ^ { \circ } .
For both geometries the best fit temperature of the soft photons is close to 15 ^ { +3 } _ { -9 } eV .
The corresponding best fit values of the hot plasma temperature and optical depth are kT _ { e } \simeq 250–260 keV and \tau \simeq 0.16–0.37 for the slab and hemisphere respectively .
These values are substantially different from those derived fitting the data with a power-law + cut off approximation to the Comptonization component ( kT _ { e } \lower 3.225 pt \hbox { $ \sim$ } \hbox to 0.0 pt { \raise 1.161 pt \hbox { $ < $ } } 60 keV , \tau \simeq 2.4 ) .
In particular the temperature of the hot electrons estimated from Comptonization models is much larger .
This is due to the fact that accurate Comptonization spectra in anisotropic geometries show ” intrinsic ” curvature which reduces the necessity of a high energy cut-off .
The Comptonization parameter derived for the slab model is larger than predicted for a two phase plane parallel corona in energy balance , suggesting that a more “ photon-starved ” geometry is necessary .
The case of a hemispheric corona is consistent with energy balance but requires a large reflection component .
The spectral softening detected during a flare which occurred in the central part of the observation corresponds to a decrease of the Comptonization parameter , probably associated with an increase of the soft photon luminosity , the hard photon luminosity remaining constant .
The increased cooling fits in naturally with the derived decrease of the coronal temperature kT _ { e } in the high state .