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

TL;DR

This study investigates the intrinsic anisotropies in the angular distributions of gamma-ray bursts, revealing a significant anisotropy in the 'intermediate' subclass while others remain isotropic, using spherical harmonic analysis.

Contribution

It provides the first detailed analysis of intrinsic anisotropies in GRB subclasses using spherical harmonics and BATSE data, highlighting a significant anisotropy in the 'intermediate' class.

Findings

The 'intermediate' GRB subclass shows a significant intrinsic anisotropy at 97% confidence.

The full GRB sample and other subclasses are consistent with isotropy.

Spherical harmonic analysis effectively detects intrinsic anisotropies in gamma-ray burst distributions.

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 $\alpha$, any non-zero spherical harmonic proportional either to $P_n^m(\sin \delta) \sin m \alpha$ or to $P_n^m(\sin \delta) \cos m\alpha$ with $m \neq 0$ ($\delta$ 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 $P_3^1(\sin \delta) \sin m \alpha$ harmonic. The remaining two subclasses, and the full sample of GRBs, remain isotropic.