OSSE OBSERVATIONS OF BLAZARS

Results are reported on observations obtained with the Oriented Scinti llation Spectrometer Experiment (OSSE) instrument on the Compton Gamma Ray Observatory (CGRO) of 17 active galactic nuclei (AGNs) known to e xhibit blazar properties at other wavelengths. These observations span the period from 1991 June through 1994 May. Of the 33 high-confidence EGRET detections of blazars during CGRO observing phases 1 and 2 (199 1 May 16-1993 September 7), OSSE has observed eight and detected five, namely 3C 273, 3C 279, PKS 0528+134, CTA 102, and 3C 454.3. Additiona lly, OSSE has detected the BL Lac sources H1517+65.6 and PKS 2155-304, which were not detected with EGRET. Variability in the energy band 50 -150 keV is observed for all of the detected AGNs. The OSSE blazar sou rces are all well described by simple power-law models with photon num ber indices, Gamma, varying from 1.0 to 2.1 among sources. When combin ed with available, although not necessarily contemporaneous, COMPTEL a nd EGRET observations, four to five detected blazars show clear eviden ce for spectral breaks between the hard X-ray and medium-energy gamma- ray bands. The exception is the combined OSSE/EGRET data for 3C 279 du ring 1991 October, where a simple power law with Gamma approximate to 1.9 works equally well. Gamma-ray evidence for beaming in CTA 102, PKS 0528+134, and 3C 454.3 is presented.