0320013

Recently we published the first all-sky map at 511 keV (Knoedlseder et al. 2004) which clearly showed that the positron annihilation radiation from our galaxy is concentrated around the galactic center. The observed emission could not be unambiguously decomposed into bulge halo and disk components. We propose to resolve the ambiguity concerning a halo component by observing at intermediate latitudes (b=+/-25deg) North and South of the galactic center for 2*10^6 s each. Any 511 keV line emission from these regions is dominated by the halo component. The proposed observations are therefore ideally suited to establish the existence of or to setstringent limits on a galactic halo component. The existence or non-existence of a halo component has far-reaching implications for the annihilation rate and the origin of positrons in our galaxy in general and in the bulge and disk in particular. Possible halo positron sources are members of the old stellar population (such asType~Ia supernovae novae or low-mass X-ray binaries) or light dark matter. [truncated! Please see actual data for full text]