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 galaxy should be observable with future space missions such as INTEGRAL and EXIST, provided that the gas surrounding the GRB source has the typical density of the interstellar medium, < 1 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 non-relativistic ejecta until the ejecta become rarefied and the annihilation time becomes long. Second, the GRB remnant remains sufficiently hot (T > 5 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 galaxy.