Model-independent requirements for the source of positrons in the galactic centre

Model-independent requirements for the positron source in the galactic centre are formulated. From the known physical processes of positron production the most probable seems to be the e ⁺ – e ^– pair production as a result of photon-photon collisions. When certain conditions are satisfied, the efficiency of positron creation due to this mechanism can reach values 10%, which is comparable with the observed ratio of the annihilation line photon luminosity to the continuum one at E > 511 keV. Such a situation can be realized: (i) in a thermal pair-dominated mildly relativistic plasma, and (ii) on the development of a nonthermal electromagnetic cascade, initiated by relativistic particles in the field of ambient X-rays. Future gamma-ray observations at ultrahigh energies can be crucial to the choice of the model.