ACCELERATION EFFICIENCY IN NONTHERMAL SOURCES AND THE SOFT GAMMA-RAYSFROM NGC-4151 OBSERVED BY OSSE AND SIGMA

We show that the recent observations of the Seyfert galaxy NGC 4151 in hard X-rays and soft gamma-rays by the OSSE and SIGMA detectors on bo ard CGRO and GRANAT, respectively, are well explained by a nonthermal model with acceleration of relativistic electrons at an efficiency of less than or similar 50% and with the remaining power dissipated therm ally in the source (the standard nonthermal e+/- pair model assumed 10 0% efficiency). Such an acceleration efficiency is generally expected on physical grounds. The resulting model unifies previously proposed p urely thermal and purely nonthermal models. The pure nonthermal model for NGC 4151 appears to be ruled out. The pure thermal model gives a w orse fit to the data than our hybrid nonthermal/thermal model.