INTEGRAL/IBIS search for e-e+ annihilation radiation from the Galactic   Center Region
  Electron-positron annihilation radiation from the Galactic Center region has been detected since the seventies, but its astrophysical origin is still a topic of a scientific debate. We have analyzed data of the gamma-ray imager IBIS/ISGRI onboard of ESA's INTEGRAL platform in the e$^{-}$e$^{+}$ line. During the first year of the missions Galactic Center Deep Exposure no evidence for point sources at 511 keV has been found in the ISGRI data; the $2 \sigma$ upper limit for resolved single point sources is estimated to be $1.6\times 10^{-4} ph cm^{-2} s^{-1}$. 
G. De Cesare, A. Bazzano, F. Capitanio, M. Del Santo, V. Lonjou, L.   Natalucci, P. Ubertini, P. Von Ballmoos