  By dividing a sample of 48 long-duration gamma-ray bursts (GRBs) into four
groups with redshift from low to high and fitting each group with the Amati
relation log Eiso = a + b log Epeak, I find that parameters a and b vary with
the mean redshift of the GRBs in each group systematically and significantly.
The results suggest that GRBs evolve strongly with the cosmic redshift and
hence are not standard candles.
