OSSE has shown conclusively, for the first time, that the high-energy emission from the Seyfert class of Active Galactic Nuclei exhibits a high-energy cutoff above a few hundred keV. This is seen in NGC 4151, the brightest radio-quiet AGN in the OSSE energy band, as well as the average of all Seyfert AGNs detected by OSSE. Above 50 keV an exponentially-cutoff power-law spectrum, N(E) µ EG exp(E/Ec), is an adequate description of the observed spectrum. For the average of all NGC 4151 observations, we find a power-law index, G = 1.83 ± 0.03, and cutoff energy, Ec = 96 ± 43 keV; for the average Seyfert we find G = 1.5 ± 0.3 and Ec = 120 ± 37 keV. Combining the OSSE data with lower energy observations provides the best insight to the nature of these sources. These analyses of OSSE, Ginga and EXOSAT data suggest that the best physical model for radio-quiet Seyferts is mildly relativistic thermal Comptonization (including a Compton-reflection component, required by the X-ray data). The best fit plasma parameters are kT ~ 100 keV and optical depth, t ~ 1.
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