DETECTION OF SIGNATURE CONSISTENT WITH COSMOLOGICAL TIME DILATION IN GAMMA-RAY BURSTS

If gamma-ray bursters are at cosmological distances-as suggested by th eir isotropic distribution on the sky and by their number-intensity re lation-then the burst profiles will be stretched in time, by an amount proportional to the redshift, 1 + Z. We have tested data from the Com pton Gamma Ray Observatory's Burst and Transient Source Experiment (BA TSE) for such time dilation. Out of 590 bursts observed by BATSE, 131 bursts were analyzed; bursts with durations shorter than 1.5 s were ex cluded. We used three tests to compare the timescales of bright and di m bursts, the latter, on average, being more distant than the former. Our measures of timescale are constructed to avoid selection effects a rising from intensity differences by rescaling all bursts to fiducial levels of peak intensity and noise bias. (1) We found that the total r escaled count above background for the dim burst ensemble is approxima tely twice that for the brightest bursts-translating into longer durat ions for the dim bursts. (2) Wavelet-transform decompositions of the b urst profiles confirmed at this dilation operates over a broad range o f timescales. (3) Structure on the shortest timescales was examined us ing a procedure which aligns the highest peaks of profiles from which the noise has been optimally removed using a wavelet threshold techniq ue. In all three tests, the dim bursts are stretched by a factor of ap proximately 2 relative to the bright ones, over seven octaves of times cale. We calibrated the measurements by dilating synthetic bursts that approximate the temporal characteristics of bright BATSE bursts. Resu lts are consistent with bursts at BATSE's peak-flux completeness limit being at cosmological distances corresponding to Z approximately 1, a nd thus with independent cosmological interpretations of the BATSE num ber-intensity relation. Alternative explanations of our results, arisi ng from the nature of physical processes in bursts, are still possible .