Testing Comptonizing Coronae on a LongBEPPOSAXObservation of the Seyfert 1 Galaxy NGC 5548

We test accurate models of Comptonization spectra over the high quality dataof the BeppoSAX long look at NGC 5548, allowing for different geometries of thescattering region, different temperatures of the input soft photon field anddifferent viewing angles. We find that the BeppoSAX data are well representedby a plane parallel or hemispherical corona viewed at an inclination angle of30$^{\circ}$. For both geometries the best fit temperature of the soft photonsis close to 15$^{+3}_{-9}$ eV. The corresponding best fit values of the hotplasma temperature and optical depth are $kT_{\rm e}\simeq$ 250--260 keV and$\tau\simeq$ 0.16--0.37 for the slab and hemisphere respectively. These valuesare substantially different from those derived fitting the data with apower-law + cut off approximation to the Comptonization component ($kT_{\rme}\lta$ 60 keV, $\tau\simeq$ 2.4). This is due to the fact that accurateComptonization spectra in anisotropic geometries show "intrinsic" curvaturewhich reduces the necessity of a high energy cut-off. The Comptonizationparameter derived for the slab model {is} larger than predicted for a two phaseplane parallel corona in energy balance, suggesting that a more``photon-starved'' geometry is necessary. The spectral softening detectedduring a flare which occurred in the central part of the observationcorresponds to a decrease of the Comptonization parameter, probably associatedwith an increase of the soft photon luminosity, the {hard} photon luminosityremaining constant.Comment: 36 pages, 9 figures, accepted by Ap