COMPTON-BACKSCATTERED ANNIHILATION RADIATION FROM THE GALACTIC-CENTERREGION

On 1989 May 22, the High Energy X-ray and Gamma-ray Observatory for Nu clear Emissions (HEXAGONE), a balloon-borne high-resolution germanium spectrometer with an 18-degrees field of view, observed the Galactic C enter (GC) from 25 to 2500 keV. The GC photon spectrum is obtained fro m the count spectrum by a model-independent method which accounts for the effects of passive material in the instrument and scattering in th e atmosphere. Besides a positron annihilation line with a flux of (10. 0 +/- 2.4) x 10(-4) photons cm-2 s-1 and a full width at half-maximum (FWHM) of (2.9 + 1.0, -1.1) keV, the spectrum shows a peak centered at (163.7 +/- 3.4) keV with a flux of (1.55 +/- 0.47) x 10(-3) photons c m-2 s-1 and a FWHM of (24.4 +/- 9.2) keV. The energy range 450-507 keV shows no positronium continuum associated with the annihilation line, with a 2 sigma upper limit of 0.90 on the positronium fraction. The 1 64 keV feature is interpreted as Compton backscatter of broadened and redshifted annihilation radiation, possibly from the source 1E 1740.7- 2942. A minimum unscattered annihilation line flux of 0.026 photons cm -2 s-1 would have been required to create this feature, or approximate ly 10(44) annihilations/s at the GC. Although this flux is not seen in our observation, its absence can be explained by occultation of the u nscattered flux, by optically thick material or by light-travel time d elay of the scattered flux.