TESTING ISOTROPY VERSUS CLUSTERING OF GAMMA-RAY BURSTS

The cosmological origin for gamma-ray bursts has been gaining popularity primarily because they manifest nearly isotropic angular distribution. Analyses of the data using different statistical tests have claimed contradictory results on the degree and significance of deviation of the angular distribution from isotropy. In this paper we apply several tests, which we believe are more powerful than those applied previously, to the BATSE 1B and 2B catalogs. In the 1B catalog we find that the excess of sources at small angles claimed by Quashnock & Lamb (1993b) is significant and can be interpreted as evidence that 15 sources have repeated twice. We also find some weak evidence for clustering of sources on a scale of about 30 degrees. These features, however, are absent in the 2B catalog. We also investigate dependence of the angular distribution on burst intensity and find no strong correlation or deviation from isotropy. In a separate paper (Petrosian & Efron 1995) we search for a correlation between temporal and angular distribution of the bursts and find some marginal evidence for repeaters in both the 1B and 2B catalogs.