Repeating Sources of Classical Gamma-Ray Bursts

From an analysis of the first catalog of the BATSE experiment (Fishman et al. 1993,1994a) on the Compton Gamma Ray Observatory, we find an excessive number of pairs of gamma-ray bursts which are clustered in both space and time. The angular separation between the two bursts in each pair is less than their positional uncertainties, and the interval between their occurrence times is within several days. Optimizing the signal, we find that the probability of observing such a clustered excess from a Poisson ensemble is ~2 x 10^-5^. We suggest that these bursts arise from repeating sources. A detailed study of the most prolific source, GBS 0855-00, is presented in a separate paper (Wang & Lingenfelter 1993a, b). Unlike most of the "soft" gamma-ray repeaters, these repeating bursts have relatively hard spectra, complex light curves, and widely varying durations, that are indistinguishable from classical gamma-ray bursts. Although the significance of the clustered excess was optimized a posteriori, because it depends on temporal and spatial bin sizes that could not be defined a priori, we can use the optimizations from the first catalog to test subsequent BATSE data sets. Unfortunately, the failure of the on-board tape recorders during the second catalog period seriously reduced the number of accurately positioned bursts (Fishman et al. 1994b), so that we can neither confirm, nor refute, the predicted repetition in that sample, and we must await the results of the third catalog.