Gamma-ray bursts and the origin of galactic positrons

A recent observation of the 511 keV electron-positron annihilation line from the Galactic bulge has prompted a debate on the origin of the galactic positrons responsible for this emission. Assuming equilibrium between annihilation and injection of positrons in the Galaxy, we investigate the possibility that positrons were produced by past gamma-ray bursts (GRBs). We compare the positron annihilation rate inferred by the observed 511 keV line, with the predicted injection rate due to e+e− pairs leaking out of individual GRBs and those pair produced by GRB photons in the dense molecular clouds at the Galactic center. We conclude that the proposed scenario is marginally consistent with observations, and can reproduce the observed normalization of the annihilation line only for higher-than-average values of the GRB rate in the Galaxy and/or of the molecular clouds optical depth for pair production.