KEY RESULTS FROM THE ORIENTED SCINTILLATION SPECTROMETER EXPERIMENT

Key results obtained by the OSSE instrument during the first four year s of the Compton Gamma Ray Observatory mission are presented. OSSE has undertaken extended observations of the gamma ray emission from the g alactic center region and found the positron annihilation radiation to be consistent with a two-component model: a spheroidal component loca ted at the galactic center and a weaker galactic disk component. Simul taneous observations with the SIGMA imaging instrument have provided t he first low-energy gamma ray spectrum of the diffuse continuum emissi on from the galactic center region. Results on galactic sources includ e the spectral observations of two new rotation-powered pulsars, PSR 1 509-58 and Vela, the discovery of 110 keV cyclotron emission from the Be X-ray binary A0535+26, and the discovery that galactic black hole t ransients have two spectral classes: thermal and power-law. Extragalac tic sources also have two spectral classes. Seyfert galaxies are typif ied by thermal spectral with exponential cutoffs from 50 to several hu ndred keV, while blazars exhibit power-law spectra that extend into th e EGRET energy range. Blazar spectra also often have a spectral break in the MeV region. OSSE has also obtained several observations of supe rnovae, including the first detection of Co-57 from SN 1987A, hard X-r ay emission from a shock-heated pre-SN wind in SN 1993J, and upper lim its for Ti-44 and Co-56 emission from Cas A and SN 1991T resp. Finally , recent observations to confirm the COMPTEL observation of 4.4 and 6. 1 MeV line emission from the Orion region have provided only upper lim its, thereby placing constraints on the intensity and/or distribution of the emission.