OSSE observations of NGC 4151

We report results of a two‐week observation of NGC 4151 with the OSSE instrument. The source had a very soft spectrum which falls off exponentially with an e‐folding energy of 39.2±1.5 keV in the energy range 65–800 keV, and had an intensity at 100 keV of 2.33±0.05×10−2 photons cm−2 sec−1 MeV−1. Simple pair cascade models in a compact source, which have been suggested for Seyfert galaxies, cannot explain the steepness and shape of the spectrum above 100 keV. We suggest pair loading and reacceleration as the mechanism responsible for the observed X‐ray and gamma‐ray spectra and the previously reported luminosity—spectral index correlation between 2–20 keV.