If gamma‐ray bursters are at cosmological distances ‐ a possibility suggested by their isotropic distribution and radial spatial inhomogeneity ‐ then the temporal profiles of more distant sources will be time dilated compared to those of relatively nearby sources. We apply two brightness‐independent tests for time dilation. Selection effects arising from intensity differences are removed by rescaling all bursts and associated noise levels to a canonical dim peak intensity. The first test measures the total normalized flux above background and indicates that dim bursts have approximately twice as much temporal structure as do bright bursts, suggesting that dim bursts are longer. Using wavelet transforms we show that, on average, dim bursts have significantly more structure than bright bursts on all time scales from 128 ms to 64 s.
Simulations that approximate actual burst profiles indicate that time dilation should be detectable, given the observed distribution of burst durations and the assumption of a modest range in intrinsic luminosity. The results from analysis of about 100 BATSE bursts are consistent with cosmological distances, i.e., with a relative time dilation factor, between brightest and dimmest burst, of (Z+1)∼1.5–3.
