Superlong gamma-ray bursts

We searched for anomalously long gamma-ray bursts (GRBs) in the archival records of the Burst and Transient Sources Experiment (BATSE). Ten obvious superlong (>500 s) GRBs with almost continuous emission episodes were found. Nine of these events were known from the BATSE catalog, but five had no duration estimates; we found one burst for the first time. We also detected events with emission episodes separated by a long period of quiescence (up to ∼1000 s) with a total duration of 1000–2000 s. In the latter case, we cannot reach an unequivocal conclusion about a common origin of the episodes due to the BATSE poor angular resolution. However, for most of these pairs, the probability of independent GRBs coinciding is much lower than unity, and the probability that all of these are coincidences is ∼10−8. All of the events have a hardness ratio (the ratio of the count rates in different energy channels) typical of GRBs, and their unique duration is unlikely to be related to their high redshifts. Superlong bursts do not differ in their properties from typical long (>2 s) GRBs. We estimated the fraction of superlong GRBs (>500 s) among the long (>2 s)GRBs in the BATSE sample with fluxes up to 0.1 ph cm−2 s−1 to be between 0.3 and 0.5%, which is higher than the estimate based on the BATSE catalog.