DISK PLUS HALO MODELS OF GAMMA-RAY BURST SOURCES

We show that, in principle, almost-equal-to 70% of the gamma-ray bursts observed by BATSE can come from local (less than or similar to kpc), Galactic disk neutron stars, with the rest in an extended Galactic halo. We consider three possible forms for the distribution of the Galactic halo sources: a Gaussian halo, an exponential halo, and a standard ''dark matter'' halo. We find that for the Gaussian halo the fraction of bursts that can come from the Galactic disk can be almost-equal-to 2/3, close to the maximum possible value; for exponential and dark matter halos, the fraction can be almost-equal-to 1/2 and almost-equal-to 1/5, respectively. As examples, we consider two particular disk-halo combinations. In both, we use an exponential disk with scale height z0, and BATSE can detect disk sources to a distance D(d) = (213)z0. In the first example, we combine this with a Gaussian halo whose peak is at R(s) = 25 kpc, and sigma = 38 kpc. The Earth is displaced R0 = 8.5 kpc from the Galactic Center, and BATSE can detect halo sources to a distance D(h) = 200 kpc. We take 66.8% of the sources to be in the disk. For the second example, we use a '' dark matter'' halo that has a ''core radius'' R(c) = 22.5 kpc, R0 = 8.5 kpc, and D(h) = 135.0 kpc; 20.2% of the sources are taken to be in the disk. In each case, the values of [V/V(max)] [sin2 b], [cos theta], and the C(max)/C(min) distribution are all easily consistent with the BATSE observations. Dividing the bursts into three, equal-sized groups of the brightest, intermediate, and faintest, there is little difference in the values of [sin2 b] and [cos theta], agreeing with the BATSE observations. The disk sources have luminosities approximately 10(36-37) ergs s-1, while those in the halo have luminosities approximately 10(41-42) ergs s-1. The brightest observed bursts must come from the halo; this suggests that the bursts that exhibit cyclotron features come from halo sources.