Galactic Models of Gamma-Ray Bursts

Gamma-ray bursts (GRBs) continue to confound astrophysicists nearly a quarter century after their discovery (1). Before the launch of CGRO, most scientists thought that GRBs came from magnetic neutron stars residing in a thick disk (having a scale height of up to ∼ 2 kpc) in the Milky Way (2,3). The data gathered by BATSE showed the existence of a rollover in the cumulative brightness distribution of GRBs and that the sky distribution of even faint GRBs is consistent with isotropy (4,5). This rules out a thick Galactic disk source population, with (6) or without (7) spiral arms. Consequently, the primary impact of the BATSE results has been to intensify debate about whether the bursts are Galactic or cosmological in origin. Galactic models attribute the bursts primarily to high-velocity neutron stars in a Galactic corona, which must extend one sixth or more of the distance to Andromeda (dM31 ∼ 690 kpc) in order to avoid any discernible anisotropy (8,9). Cosmological models place the GRB sources at distances d ∼ 1 − 3 Gpc, corresponding to redshifts z ∼ 0.3 − 1. A source population at such large distances naturally produces an isotropic distribution of bursts on the sky, and the expansion of the universe or source evolution can reproduce the observed rollover in the cumulative brightness distribution (10). Within the context of this workshop, we focus on Galactic corona models involving high velocity neutron stars. A recent discussion of cosmological models may be found in, e.g., Blaes (11).