ZHANG B.-B., ZHANG B., MURASE K., CONNAUGHTON V. and BRIGGS M.S.
Abstract (from CDS):
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 tburst based on both γ-ray and X-ray light curves rather than using γ-ray observations alone. We find that tburstcan be reliably measured in 343 GRBs. Within this "good" sample, 21.9% GRBs have tburst ≳ 103 s and 11.5% GRBs have tburst ≳ 104 s. There is an apparent bimodal distribution of tburstin this sample. However, when we consider an "undetermined" sample (304 GRBs) with tburst possibly falling in the gap between GRB duration T90and the first X-ray observational time, as well as a selection effect against tburstfalling into the first Swift orbital "dead zone" due to observation constraints, the intrinsic underlying tburstdistribution 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.
gamma-ray burst: general
VizieR on-line data:
<Available at CDS (J/ApJ/787/66): table1.dat>