GRIS OBSERVATIONS OF POSITRON-ANNIHILATION RADIATION FROM THE GALACTIC-CENTER

We report new observations of the Galactic center positron annihilation source in which, for the first time, the 511 keV line has been spectrally resolved and the Galactic plane and center components have been independently measured. High-resolution spectra were obtained of the Galactic center with the Gamma-Ray Imaging Spectrometer (GRIS) during May and October of 1988 balloon flights. The 511 keV line source was found to be turned on again in October of 1988, at a flux of (11.8 +/- 1.6) x 10(-4) photons cm-2 s-1, after being observed in a quiescent state since the early 1980s. There is evidence for variability between May and October. An observation of a point in the Galactic plane 25-degrees west of the center was made during the October flight. The 511 keV line flux for this pointing was significantly lower, less than 4.0 x 10(-4) photons cm-2 s-1 (2-sigma upper limit), than that for the center pointing, proving unambiguously that the center emission is concentrated near l(II) = 0-degrees. The GRIS data combined with that from other narrow-field observations in the 1980s support the two-component model of the positron annihilation source. The best fit to these data sets gives a flux of approximately 8 x 10(-4) photons cm-2 s-1 for the center point source component in its high state. A strong hard X-ray continuum was detected in the Galactic plane observation. This "diffuse" continuum component makes up the difference between the observed continuum in the GRIS Galactic center pointing and the known point source contributions, solving the long-standing mystery of why wide-field instruments have detected such high continuum emission from the Galactic center.