Testing for Extragalactic Correlations of Gamma-Ray Bursts

Abstract

If gamma-ray bursts (GRBs) are cosmological they need not necessarily be spatially coincident with a host galaxy--in many cosmological models the progenitors are ejected to large distances from their parent galaxies. While the optical transient of GRB 970228 may be close to a faint galaxy, there are several other galaxies in the field which could be the host if the progenitors are ejected to ~50 kpc. If GRBs come from such a widely distributed progenitor population then it should be possible to search for a statistical correlation between optical transients and nearby galaxies. We provide a statistical framework for quantifying a correlation between potential host galaxies and GRBs, taking into account that galaxies are clustered. From simulations, we estimate that if GRBs occur within l < 100 kpc of their host, then optical observations B < 24.0 of the galaxy field near just 6 optical transient positions are required to find a correlation with potential hosts at the 95 percent confidence level. The methodology constructed herein is extendible to any cross-correlation analysis and is especially useful if either set of objects is clustered. Ultimately, if the distance scale to GRBs is established as cosmological, this analysis can be used to determine the spatial distribution of GRBs near their host galaxies.