Evidence for the Galactic Origin of Gamma-Ray Bursts

Abstract

We investigate the angular distribution of the $\gamma$-ray bursts in the publicly available BATSE catalogue, using the measures of burst brightness $B$ and short time scale ($\simless$ 0.3 s) variability $V$ which we introduced earlier. We show that the 54 type I ($\log V \le -0.8$) bursts lying in the middle brightness range 490 counts $\le B \le$ 1250 counts (corresponding to 1/3 of all type I bursts) exhibit a Galactic dipole moment of $\langle \cos \theta \rangle = 0.204 \pm 0.079$ {\it and} a deviation of the Galactic quadrupole moment from 1/3 of $\langle \sin^2 b \rangle - 1/3 = -0.104 \pm 0.041$. Using Monte Carlo simulations which include the BATSE sky exposure map and taking into account division of the type I bursts into three equal samples, we find that the probability by chance of an isotropic distribution of 54 bursts exhibiting values of $\langle \cos \theta \rangle$ {\it and} the negative of $\langle \sin^2 b \rangle - 1/3$ which equal or exceed the observed values is $6.6 \times 10^{-5}$. We conclude that $\gamma$-ray bursts are Galactic in origin.