Long-term monitoring of NGC 4151 by OSSE

We report results of over 5 yr monitoring of the Seyfert galaxy NGC 4151 by the Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory. A total of more than 139 days of observations in the energy band 50 keV-10 MeV was obtained in 11 viewing periods. These observations show a photon spectrum in the 50-800 keV band that is well described by an exponentially cutoff power law of the form Phi(E) proportional to E(-1.6)e(-E/100 keV). Long-term luminosity changes of no more than a factor of 2 are observed with only modest change in the spectral shape. With these data we set the most sensitive limit to date on positron annihilation radiation from a Seyfert active galactic nucleus and address the role of e(+/-) pair production and nonthermal processes in this source.