TESTING ISOTROPY VERSUS CLUSTERING OF GAMMA-RAY BURSTS

The cosmological origin for gamma-ray bursts has been gaining populari ty primarily because they manifest nearly isotropic angular distributi on. Analyses of the data using different statistical tests have claime d contradictory results on the degree and significance of deviation of the angular distribution from isotropy. In this paper we apply severa l tests, which we believe are more powerful than those applied previou sly, to the BATSE 1B and 2B catalogs. In the 1B catalog we find that t he excess of sources at small angles claimed by Quashnock & Lamb (1993 b) is significant and can be interpreted as evidence that 15 sources h ave repeated twice. We also find some weak evidence for clustering of sources on a scale of about 30 degrees. These features, however, are a bsent in the 2B catalog. We also investigate dependence of the angular distribution on burst intensity and find no strong correlation or dev iation 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 1 B and 2B catalogs.