Identifying Gamma-Ray Burst Remnants through Positron Annihilation Radiation

Abstract

We model the annihilation of relic positrons produced in a gamma-ray burst (GRB) after its afterglow has faded. We find that the annihilation signal from at least one GRB remnant in the Milky Way should be observable with future space missions such as the International Gamma-Ray Astrophysical Laboratory and the Energetic X-ray Imaging Survey Telescope, provided that the gas surrounding the GRB source has the typical density of the interstellar medium, less than or similar to1 cm(-3). Three fortunate circumstances conspire to make the signal observable. First, unlike positrons in a standard supernova, the GRB positrons initially travel at a relativistic speed and remain ahead of any nonrelativistic ejecta until the ejecta become rarefied and the annihilation time becomes long. Second, the GRB remnant remains sufficiently hot (Tgreater than or similar to5 x 10(5) K) for a strong annihilation line to form without significant smearing by three-photon decay of positronium. Third, the annihilation signal persists over a time longer than the average period between GRB events in the Milky Way.