DURATION DISTRIBUTIONS OF BRIGHT AND DIM BATSE GAMMA-RAY BURSTS

We have measured the T-90 and T-50 durations of bright and dim gamma-ray bursts detected by the Compton Gamma-Ray Observatory's Burst and Transient Source Experiment (BATSE). The T-90 (T-50) duration is defined as the interval over which 5% (25%) to 95% (75%) of the burst counts accumulate. Out of 775 bursts observed by BATSE, 159 bursts were analyzed; bursts with durations shorter than 1.5 s were excluded. A Kolmogorov-Smirnov test yields a probability of 6 x 10(-5) that the T-50 durations of the dim and bright samples are drawn from the same parent population. We find that the centroid and extent of the duration distribution for the dim sample are scaled by approximately a factor of 2 relative to those of the bright sample. The measured time-dilation factor is not sensitive to choice of energy band. These results are quantitatively consistent with previous tests for time dilation in a smaller sample of BATSE bursts. The sources of dimmer bursts, if cosmological, would lie at redshifts of order 2.