SEARCH FOR COMPTON-BACKSCATTERED ANNIHILATION RADIATION FROM THE GALACTIC-CENTER WITH THE OSSE

An emission feature near 170 keV, interpreted as Compton-backscattered 511 keV positron-annihilation radiation, has been reported twice by b alloon-borne germanium spectrometers from within similar to 15 degrees of the Galactic center (Leventhal, MacCallum, and Stang 1978; Smith e t al. 1993). Upper limits on this feature set by HEAO 3 (Mahoney, Ling , and Wheaten 1993) and other instruments indicate that it must be tra nsient. We have searched data from the Oriented Scintillation Spectrom eter Experiment (OSSE) on the Compton Gamma Ray Observatory (CGRO) for this feature, using daily spectral accumulations from all pointings n ear the Galactic center up to 1993 August, and covering most of the re gion viewed by the balloon instruments. We find no evidence for backsc atter emission. Under the hypothesis that the source is 1E 1740.7-2942 , the OSSE data set (186 days) disagrees with the balloon measurements with 99.3% confidence. The average daily 3 sigma OSSE upper limit on backscatter flux from 1E 1740.7-2942 is 6.8 10(-4) photons cm(-2) s(-1 ), compared to the 1.3 x 10(-3) photons cm(-2) reported by the balloon observations. We also saw no evidence in 186 days for linelike emissi on from the point source EXS 1737.9-2952 recently discovered by Grindl ay, Covault, and Manandhar (1993). This source exhibited bright emissi on from 83-111 keV, which has been interpreted as doubly backscattered 511 keV radiation. The average daily 3 sigma upper limit from OSSE fo r this line is 9.8 x 10(-4) photons cm(-2) s(-1), or similar to 8% of the reported flux.