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The durations (T90) of 315 GRBs detected with Fermi/GBM (8-1000 keV) by 2011September are calculated using the Bayesian Block method. We compare the T90distributions between this sample and those derived from previous/current GRBmissions. We show that the T90 distribution of this GRB sample is bimodal, witha statistical significance level being comparable to those derived from theBeppoSAX/GRBM sample and the Swift/BAT sample, but lower than that derived fromthe CGRO/BATSE sample. The short-to-long GRB number ratio is also much lowerthan that derived from the BATSE sample, i.e., 1:6.5 vs 1:3. We measure T90 inseveral bands, i.e., 8-15, 15-25, 25-50, 50-100, 100-350, and 350-1000 keV, toinvestigate the energy-dependence effect of the bimodal T90 distribution. It isfound that the bimodal feature is well observed in the 50-100 and 100-350 keVbands, but is only marginally acceptable in the 25-50 keV and 350-1000 keVbands. The hypothesis of the bimodality is confidently rejected in the 8-15 and15-25 keV bands. The T90 distributions in these bands are roughly consistentwith those observed by missions with similar energy bands. The parameter T90 asa function of energy follows \bar T90 \propto E^{-0.20\pm 0.02} for long GRBs.Considering the erratic X-ray and optical flares, the duration of a burst wouldbe even much longer for most GRBs. Our results, together with the observedextended emission of some short GRBs, indicate that the central engine activitytime scale would be much longer than T90} for both long and short GRBs and theobserved bimodal T90 distribution may be due to an instrumental selectioneffect.

astrophysical journal/the astrophysical journal

A Comprehensive Analysis of Fermi Gamma-ray Burst Data: III.  Energy-Dependent T90 Distributions of GBM GRBs and Instrumental Selection  Effect on Duration Classification