An analysis of the durations of Swift gamma-ray bursts

Context. Swift detectors are found to be more sensitive to long-soft bursts than pre-Swift missions. This may largely bias the distribution of durations and thus classification of gamma-ray bursts. Aims. We systematically investigate the duration distribution of gamma-ray bursts in the Swift era vs. that of pre-Swift bursts. Methods. For the purpose of this study, statistical methods such as the K-S test and linear/non-linear fitting analysis have been used to examine the duration properties of Swift bursts in both observer and source frames. Results. For 95 GRBs with known redshift, we show that two log-normal distributions of duration are clearly divided at T-90 similar or equal to 2 s. The intrinsic durations also show a bimodal distribution but shift systematically toward the smaller value and the distribution exhibits a narrower width than the observed one. Swift long bursts exhibit a wider duration dynamic range in both observer and source frames in comparison to pre-Swift long bursts. Conclusions. We find that Swift bursts and pre-Swift ones can share the same criterion of classification in terms of duration at 2 s, although both monitors have large differences with respect to sensitivity of a given energy band.