GAMMA-RAY BURSTS IN THE GALACTIC HALO

THE angular and luminosity distributions of the gamma-ray bursts observed by the BATSE instrument, on the Gamma Ray Observatory satellite, cannot be explained by sources confined to the galactic plane 1. Instead the observations are consistent with a nearly isotropic source density that falls with distance. Although this permits a cosmological origin, it also permits an origin in the halo of our Galaxy, preserving some aspects of models in which galactic neutron stars are the sites of the bursts. I show here that significant isotropy can be achieved with a spherically symmetric halo model if it extends out beyond 100 kpc. Large halo core radii enhance isotropy, although consistency with observation is possible for core radii as small as 5 kpc if the halo radius is sufficiently large. The intrinsic luminosity distribution of gamma-ray bursts must be treated as a free parameter to fit the observations. If gamma-ray bursts are from the halo, they are likely to be old population II neutron stars, because models based on pulsars escaping from the galactic plane have strong anisotropies.