An observation of annihilation radiation from the Galactic Center region

The Galactic Center region was observed on 22 May 1989 with a high resolution gamma-ray spectrometer flown from Alice Springs, Australia. The instrument contained an array of 12 cooled Ge detectors, each 5.5 cm dia x 5.5 cm long, with an energy resolution of 2.2 keV at 0.5 MeV, which were collimated to 18° FWHM. The observation was made at approximately 4 gm/cm2 atmosphere depth for 6 hours in a series of target and background pointings lasting 20 minutes each. Results from multiparameter Gaussian fits to the data are: 511 keV flux = (8.9 ± 2.7) x 10−4 ph/cm2-sec, line width of 1.1 −1.1+1.4 keV, and < 3.2 keV FWHM at 95% confidence. If the flux is interpreted as diffuse galactic emission, the 24° effective aperture for a uniform source gives a flux of (2.12 ± 0.64) x10−3 ph/cm2-s-rad, consistent with the SMM observations of diffuse galactic flux. The flux and width are both less than the October 1988 measurements with a 17° FWHM instrument which were obtained when the compact source at the galactic center was known to be “on”. Thus the variable compact source in the Galactic Center region may be responsible for the wider 511 keV line emission. The line width limit is consistent with annihilation in the warm (104 K) phase of the interstellar medium.