Time-variable positron annihilation radiation from the galactic center direction

Our balloon-borne germanium ..gamma..-ray telescope has been flown for a third time over Alice Springs, Australia to further study the galactic center source 511 keV annihilation-line radiation. The source was found to be in an ''off'' or ''low'' state. Our best fit experimental value for the line flux at Earth is zero, with a ''1 sigma'' upper bound of 3.8 x 10/sup -4/ photons cm/sup -2/ s/sup -1/. This null result is discussed in the context of other measurements and available theoretical models, the most promising of which involve a galactic supernova or central compact object like a massive black hole.