Search for Compton-Backscattered Annihilation Radiation from the Galactic Center with OSSE
Session 116 -- Galactic Center and Diffuse Galactic Emission
Oral presentation, Saturday, January 15, 10:15-11:45, Salon IV Room (Crystal Gateway)

[116.06] Search for Compton-Backscattered Annihilation Radiation from the Galactic Center with OSSE

D. M. Smith, M. Leventhal (UMCP), J. Tueller (NASA/GSFC), R. L. Kinzer, J. D. Kurfess, W. N. Johnson (NRL), G. Jung (USRA), W. R. Purcell (NWU), M. S. Strickman (NRL), M. P. Ulmer (NWU)

An emission feature near 170 keV, interpreted as Compton-backscattered positron annihilation radiation, has been observed twice by balloon-borne germanium spectrometers from within $\sim$15$\deg$ of the Galactic Center (Leventhal et al. 1978, ApJ 225,L11; Smith et al. 1993, ApJ 414,165). Upper limits on this feature set by HEAO-3 (Mahoney et al. 1993, A\&AS 97,159) and other instruments indicate that it must be transient. A transient backscatter feature, along with unscattered annihilation radiation, was observed from Nova Muscae by SIGMA (Goldwurm et al. 1992, ApJ 389,L79), with a duration of a few hours.

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory is particularly well-suited to search for such a feature. Since its launch in 1991, the OSSE instrument has observed the Galactic center region for $\sim$160 days. Its energy resolution at 170 keV ($\sim$24 keV) is comparable to the reported widths of the backscatter peak (12-24 keV), and its 3 $\sigma$ sensitivity, $\sim$3$\times$10$^{-4}$ photons/cm$^{2}$/s at 170 keV for a 40000 s observation, would easily allow detection of the $\sim$1$\times$10$^{-3}$ photon/cm$^{2}$/s fluxes reported by the balloon instruments. We present the results of a day-by-day search of the available data and discuss the statistical compatibility between the OSSE data and the balloon observations.