Pair annihilation in wind models of gamma‐ray bursts

In wind models of γ‐ray bursts, electron‐position pairs stream away from the neutron star surface, upscattering soft x‐ray photons to hard x‐ and γ‐ray energies. For sufficiently high source luminosities, the density of pairs required is high enough that significant pair annihilation occurs before the pairs leave the continuum‐forming region. We examine the effects of annihilation on the outflowing pair distribution using accurate (weak magnetic field) cross‐sections. We compute the annihilation feature produced and conclude it should not be easily observable for source distances ≳100 kpc. We also examine the validity of the single scattering approximation often made in continuum and dynamic cyclotron line‐formation calculations. Even with the inclusion of annihilation which removes pairs from the wind, we find that a photon is likely to scatter several times off the wind if the source is at a distance ≳1 kpc. This can significantly modify the computed spectrum.