ESCAPE OF BEAMED EMISSION FROM GAMMA-RAY BURSTS ON MAGNETIZED NEUTRON STARS

Gamma-ray bursts are observed to emit photons with energies in excess of tens of MeV. The emergence of multi-MeV photons must confront the growing evidence that gamma-ray bursts originate from neutron stars with a strong magnetic field. If the emission is roughly siotropic, photon annihilation leading to electron-positron pair production is very efficient, and most of the emission is attenuated. On the other hand, multi-MeV photons may escape to infinity if they are emitted in a beam about the local magnetic field. In this Letter, we calculate the amount of beaming required to allow high-energy photons to escape. Applying our result to the burst GB780325 which emits photons up to 6 MeV and shows evidence of a 5 × 1012 G magMeV as seen in some bursts does not pose a problem.