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2.10. Repetition?

Quashnock and Lamb [167] suggested that there is evidence from the data in the BATSE 1B catalog for repetition of bursts from the same source. If true, this would severely constrain most GRB models. In particular, it would rule out any other model based on a `once in lifetime' catastrophic event. This claim has been refuted by several authors [168, 169] and most notably by the analysis of the 2B data [170] and 3B data [171].

A unique group of four bursts - "the gang of four" - emerged from the same position on the sky within two days [68]. One of those bursts (the third burst GRB961029a) was extremely strong, one of the strongest observed by BATSE so far. Consequently it was observed by the IPN network as well, and its position is known accurately. The other three (GRB961027a, GRB961027b and GRB961029d) were detected only by BATSE. The precise position of one burst is within the 1sigma circles of the three other bursts. However, two of the bursts are almost 3sigma away from each other. Is this a clear cut case of repetition? It is difficult to assign a unique statistical significance to this question as the significance depends critically on the a priori hypothesis that one tests. Furthermore, the time difference between the first and the last bursts is less than two days. This is only one order of magnitude longer than the longest burst observed beforehand. It might still be possible that all those bursts came from the same source and that they should be considered as one long burst.