|
Description
The durations (T_90_) of 315 gamma-ray bursts (GRBs) detected with Fermi/GBM (8-1000keV) up to 2011 September are calculated using the Bayesian Block method. We compare the T_90_ distributions between this sample and those derived from previous/current GRB missions. We show that the T_90_ distribution of this GRB sample is bimodal, with a statistical significance level comparable to those derived from the BeppoSAX/GRBM sample and the Swift/BAT sample, but lower than that derived from the CGRO/BATSE sample. The short-to-long GRB number ratio is also much lower than that derived from the BATSE sample, i.e., 1:6.5 versus 1:3. We measure T_90_ in several bands, i.e., 8-15, 15-25, 25-50, 50-100, 100-350, and 350-1000keV, to investigate the energy-dependence effect of the bimodal T_90_distribution. It is found that the bimodal feature is well observed in the 50-100 and 100-350keV bands, but is only marginally acceptable in the 25-50keV and 350-1000keV bands. The hypothesis of bimodality is confidently rejected in the 8-15 and 15-25keV bands. The T_90_distributions in these bands are roughly consistent with those observed by missions with similar energy bands. The parameter T_90_ as a function of energy follows {overline}{T}_90_{propto}E^-0.20+/-0.02^ for long GRBs. Considering the erratic X-ray and optical flares, the duration of a burst would be even longer for most GRBs. Our results, together with the observed extended emission of some short GRBs, indicate that the central engine activity timescale would be much longer than T_90_ for both long and short GRBs and the observed bimodal T_90_ distribution may be due to an instrumental selection effect.
|
