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-r ay bursts detected by the Compton Gamma-Ray Observatory's Burst and Tr ansient 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 analy zed; bursts with durations shorter than 1.5 s were excluded. A Kolmogo rov-Smirnov test yields a probability of 6 x 10(-5) that the T-50 dura tions of the dim and bright samples are drawn from the same parent pop ulation. We find that the centroid and extent of the duration distribu tion for the dim sample are scaled by approximately a factor of 2 rela tive to those of the bright sample. The measured time-dilation factor is not sensitive to choice of energy band. These results are quantitat ively consistent with previous tests for time dilation in a smaller sa mple of BATSE bursts. The sources of dimmer bursts, if cosmological, w ould lie at redshifts of order 2.