An annihilation fountain at the Galactic center?
  Recently, data of taken with the OSSE experiment have been combined with scanning observations by TGRS and SMM to produce maps of the narrow Galactic 511 keV line emission (Purcell et al. 1997). A modelling of the combined data give evidence for three distinct features: the Galactic plane, a central bulge, and an extended emission region at positive latitudes above the Galactic center. It has been proposed that the high-latitude feature is associated with a fountain of radioactive debris produced by enhanced supernova activity in the Galactic center region (Dermer and Skibo 1997). Here we discuss this scenario in more detail: we have build a 2-dimensional code to follow the development of a hydrodynamical fountain in the Galactic center region. We have then calculated the transport, cooling, and annihilation of positrons as test particles in the outflow. As a result we find difficulties with the fountain model if the positrons are produced by supernovae in a starburst near the Galactic center. Annihilation on dust grains may increase the 511 keV line flux at high latitudes. Alternatively the observed positrons may not be entirely produced by supernovae. 
Martin Pohl (DSRI)