How Long does a Burst Burst?
Astrophysical Journal, 787, pp 66, 2014
Several gamma-ray bursts (GRBs) last much longer (~hours) inγ-rays than typical long GRBs (~minutes), and it has recently beenproposed that these "ultra-long GRBs" may form a distinct population,probably with a different (e.g., blue supergiant) progenitor thantypical GRBs. However, Swift observations suggest that many GRBs haveextended central engine activities manifested as flares and internalplateaus in X-rays. We perform a comprehensive study on a large sampleof Swift GRBs with X-Ray Telescope observations to investigate GRBcentral engine activity duration and to determine whether ultra-longGRBs are unusual events. We define burst duration t burstbased on both γ-ray and X-ray light curves rather than usingγ-ray observations alone. We find that t burst can bereliably measured in 343 GRBs. Within this "good" sample, 21.9% GRBshave t burst >~ 103 s and 11.5% GRBs have tburst >~ 104 s. There is an apparent bimodaldistribution of t burst in this sample. However, when weconsider an "undetermined" sample (304 GRBs) with t burstpossibly falling in the gap between GRB duration T 90 and thefirst X-ray observational time, as well as a selection effect against tburst falling into the first Swift orbital "dead zone" due toobservation 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 GRBpopulation is inconclusive, and further multi-wavelength observationsare needed to draw a firmer conclusion. We also discuss the theoreticalimplications of our results. In particular, the central engine activityduration of GRBs is generally much longer than the γ-ray T90 duration and it does not even correlate with T90. It would be premature to make a direct connection betweenT 90 and the size of the progenitor star.
gamma-ray burst; general