2007MNRAS.379L..55L

Because of the limit in the number of gamma-ray bursts (GRBs) with available redshifts and spectra, all current investigations on the correlation among GRB variables use burst samples with redshifts that span a very large range. Evolution and selection effects have thus been ignored, which might have an important influence on the results. In this Letter, we divide the 48 long-duration GRBs studied by Amati into four groups with redshift from low to high, each group containing 12 GRBs. Then we fit each group with the Amati relation logE_iso= a + blogE_peak, and check whether the parameters a and b evolve with the GRB redshift. We find that a and b vary with the mean redshift of the GRBs in each group systematically and significantly. Monte Carlo simulations show that there is only a ∼4 per cent chance that the variation is caused by the selection effect arising from the fluence limit. Hence our results may indicate that GRBs evolve strongly with cosmological redshift.