On the existence of the intrinsic anisotropies in the angular distributions of gamma-ray bursts

Abstract

This articles is concerned primarily with the intrinsic anisotropy in the angular distribution of 2281 gamma-ray bursts (GRBs) collected in Current BATSE Gamma-Ray Burst Catalog until the end of year 1998, and, second, with intrinsic anisotropies of three subclasses ("short", "intermediate", "long") of GRBs. Testing based on spherical harmonics of each class, in equatorial coordinates, is presented. Because the sky exposure function of BATSE instrument is not dependent on the right ascension $α$, any non-zero spherical harmonic proportional either to $P_n^m(\sin δ) \sin m α$ or to $P_n^m(\sin δ) \cos mα$ with $m \neq 0$ ($δ$ is the declination), immediately indicates an intrinsic non-zero term. It is a somewhat surprising result that the "intermediate" subclass shows an intrinsic anisotropy at the $97\%$ significance level caused by the high non-zero $P_3^1(\sin δ) \sin m α$ harmonic. The remaining two subclasses, and the full sample of GRBs, remain isotropic.