Time dilation in the peak-to-peak timescales of gamma-ray bursts

We present strong evidence of time stretching in the peak-to-peak timescales in the light curves of BATSE gamma-ray bursts (GRBs). Extensive tests are performed on artificially dilated bursts to verify that the procedure for extracting the peak-to-peak timescales correctly recovers the stretching of the bursts. The resulting robust algorithm is then applied to the 4B GRB database. We derive a stretching factor of 1.92 +/- 0.13 between the brightest burst group (P > 7.7 photons cm(-2) s(-1)) and the dimmest burst group (P = 1.0-1.4 photons cm(-2) s(-1)), with several independent peak-to-peak timescale definitions, and they agree within the uncertainties. Such an agreement strongly supports the interpretation of the observed time stretching as the time dilation caused by cosmological expansion rather than by physical selection effects. We fitted the result to cosmological models with Lambda = 0, Omega(0) from 0.2 to 1.0, and constrained the standard candle luminosity to be L-o = (7.0 +/- 2.0 +/- 2.7) x 10(56) photons s(-1). Our luminosity value is fully consistent with the value from the combined PVO and BATSE log N-log P curve with the BATSE bright bursts at low redshifts of z(bright) = 0.11 +/- 0.02 +/- 0.025. This luminosity fit is definitely inconsistent with the larger distance scale implied from associating burst density with star formation rates.