SIMBAD references

Several gamma-ray bursts (GRBs) last much longer (∼hours) in γ-rays than typical long GRBs (∼minutes), and it has recently been proposed that these "ultra-long GRBs" may form a distinct population, probably with a different (e.g., blue supergiant) progenitor than typical GRBs. However, Swift observations suggest that many GRBs have extended central engine activities manifested as flares and internal plateaus in X-rays. We perform a comprehensive study on a large sample of Swift GRBs with X-Ray Telescope observations to investigate GRB central engine activity duration and to determine whether ultra-long GRBs are unusual events. We define burst duration t_burst based on both γ-ray and X-ray light curves rather than using γ-ray observations alone. We find that t_burstcan be reliably measured in 343 GRBs. Within this "good" sample, 21.9% GRBs have t_burst ≳ 10³ s and 11.5% GRBs have t_burst ≳ 10⁴ s. There is an apparent bimodal distribution of t_burstin this sample. However, when we consider an "undetermined" sample (304 GRBs) with t_burst possibly falling in the gap between GRB duration T₉₀and the first X-ray observational time, as well as a selection effect against t_burstfalling into the first Swift orbital "dead zone" due to observation constraints, the intrinsic underlying t_burstdistribution is consistent with being a single component distribution. We found that the existing evidence for a separate ultra-long GRB population is inconclusive, and further multi-wavelength observations are needed to draw a firmer conclusion. We also discuss the theoretical implications of our results. In particular, the central engine activity duration of GRBs is generally much longer than the γ-ray T₉₀duration and it does not even correlate with T₉₀. It would be premature to make a direct connection between T₉₀and the size of the progenitor star.