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Observations of Seyfert 1 AGNs with the Oriented Scintillation Spectrometer Experiment 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 of thermal Comptonization to precisely 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 bremsstrahulung 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 present the best-fit single and two-component ST spectra, in addition to a thermal model where the upscattered soft photons are produced in an external accretion disk and are reprocessed by the hot plasma cloud.
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