OSSE Observations of NGC 4151

We report results of a two-week observation of the Seyfert galaxy NGC 4151 with the Oriented Scintillation Spectrometer Experiment (OSSE) instrument on the Compton Gamma Ray Observatory (GRO). The source had a very soft spectrum which is best represented by a broken power law or Sunyaev-Titarchuk thermal Comptonization model. It can be characterized by an exponential shape with an e-folding energy of 392 keV in the energy range 65 ? 300 keV, and an intensity at 100 keV of (2:33 0:05) 10 ?5 photons cm ?2 s ?1 keV ?1. Source intensity variability of 25% was observed during the 14 days of our observations; no signiicant change in spectral shape was detected for this period. The steepness and shape of the spectrum above 100 keV rule out (2 probability 7 10 ?4) simple pair cascade models in a compact source which have been suggested for Seyfert galaxies.