On the galactic distribution of gamma-ray bursts

Abstract

Quashnock & Lamb (hereafter QL) defined a sub‐sample of Gamma Ray Bursts (GRBs) from the publicly availably BATSE database which shows clumping toward the galactic plane, and they concluded that all GRBs are galactic in origin. The selection of these bursts involved a peak count‐rate [B, in counts (1024 ms)−1] which is uncorrected for aspect. Using, as limits, the corresponding peak fluxes (in photons cm−2 s−1) for the bursts in the QL sample, we find an additional 24 bursts, which we include in a new sample (Sample 2). We assert that the peak flux of a burst is physically more meaningful than peak count‐rate, as used by QL. We find that the significance of deviation from isotropy due to a possible galactic population in Sample 2 is much less than QL’s sample, which does not support QL’s interpretation of the anisotropies as being due to a galactic population. To make meaningful statistical statements regarding isotropy, burst samples must have peak fluxes above a minimum flux (Ipeak,LL1024), which is set by the requirement that a burst be detectable from any direction (above the horizon) with respect to GRO, at any detection threshold at which a burst was observed in that sample. Approximately 1/3 of the bursts in the QL sample have fluxes below Ipeak,LL1024. We split our Sample 2 into two sub‐samples (Sample 3 and Sample 4) which have fluxes below and above Ipeak,LL1024, respectively. We find that Sample 4 has a marginal (2.6σ) deviation from isotropy, which we consider insufficient to justify the claim that GRBs are galactic in origin.