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Observations of Seyfert 1 AGNs with the Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Observatory show a spectral softening at photon energies > several tens of keV. In the case of NGC 4151, the brightest hard X-ray Seyfert, the spectral shape in the OSSE energy range (>60 keV) is well described by the Sunyaev-Titarchuk (ST) spectral form. We use a Monte Carlo simulation to specify the limitations of the ST analysis due to the breakdown of the diffusion approximation at low optical depths, the Klein-Nishina corrections in the Compton scattering cross section at high temperatures, and the presence of bremsstrahlung radiation. We fit the combined 2 keV -1 MeV spectrum of NGC 4151 observed with Ginga (1991 May 31-June 2) and OSSE (1991 June 28-July 2) with a thermal model. We also treat a model where the upscattered soft photons are produced in an external accretion disk and are reprocessed by the hot plasma cloud. Calculations of the angle-dependent emergent radiation are presented.
