A Possible Explanation for the Peculiar Correlations in the Angular Distribution of Gamma-Ray Bursts

Abstract

It has been recently discovered that the angular autocorrelation function of gamma-ray bursts exhibits sharp peaks at angular separations of $\lesssim 4°$ (Quashnock and Lamb 1993), and at $\gtrsim 176°$ (Narayan and Piran 1993). While an excess of very close pairs of bursts can naturally arise from burst repetition or from a spatial correlation of the burst sources, a physical explanation for an angular correlation on a scale of $\simeq 180°$ seems inconceivable. We show that both sharp peaks in the correlation function can be explained by a possible bias in the determination of the burst positions. A generic way is described in which the suggested bias can be introduced into the burst localization procedure, either through instrumental imperfection or through the software analyses. We apply Monte Carlo simulations to show that the observed correlation function can be reproduced by the suggested effect. We demonstrate that the results can nicely agree with the observations even if only a fraction of the bursts are subject to the bias. We emphasis that the only motivation for suggesting the existence of this bias are the features found in the angular autocorrelation function. It does not rule out the possibility that bursts repeat. The natural way in which such bias, if it exists, explains both sharp peaks, and the various conceivable causes for the origin of this bias, make the bias hypothesis worth considering.