Restrictions on the injection energy of positrons annihilating near the Galactic center

Abstract

Beacom and Yüksel (2006) suggested an elegant method to estimate an initial energy of annihilating positrons from the ratio of 511 keV line and in-flight annihilation fluxes. The idea was that in the case of stationary injection of positrons at high energies the fluxes of 511 keV line emission (generated by thermalized positrons) and of in-flight annihilation (produced by fast positrons) were proportional to each other. For the delta-function injection spectrum they showed that the initial energy of annihilating positrons cannot exceed 3 MeV. Otherwise the expected in-flight annihilation flux of fast positrons in the range 1 to 10 MeV is higher than observed by COMPTEL. However, the conclusion may be changed significantly if one analyzes the spatial distribution of the 511 keV emission and 1-10 MeV emission. In addition if the magnetic field in Galactic center region is 1 mG or higher the injection energy of positrons produced by central compact source can be unrestricted.