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Description
Several gamma-ray bursts (GRBs) last much longer (~hours) in {gamma}-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 {gamma}-ray and X-ray light curves rather than using {gamma}-ray observations alone. We find that t_burst_can be reliably measured in 343 GRBs. Within this "good" sample, 21.9% GRBs have t_burst_>~10^3^ s and 11.5% GRBs have t_burst_>~10^4^ s. There is an apparent bimodal distribution of t_burst_ in this sample. However, when we consider an "undetermined" sample (304 GRBs) with t_burst_ possibly falling in the gap between GRB duration T_90_ and the first X-ray observational time, as well as a selection effect against t_burst_ falling into the first Swift orbital "dead zone" due to observation constraints, the intrinsic underlying t_burst_ distribution 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 {gamma}-ray T_90_ duration and it does not even correlate with T_90_. It would be premature to make a direct connection between T_90_ and the size of the progenitor star.
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