A complexity-brightness correlation in gamma-ray bursts

We observe strong correlations between the temporal properties of gamma-ray bursts (GRBs) and their apparent peak brightness. The strongest effect (wi th a significance level of similar to 10(-6)) is the difference between the brightness distributions of simple bursts (dominated by a single smooth pu lse) and complex bursts (consisting of overlapping pulses). The latter has a break at a peak flux of similar to 1.5 photons cm(-2) s(-1), while the di stribution of simple bursts is smooth down to the BATSE threshold. We also observe brightness-dependent variations in the shape of the average peak-al igned time profile (ATP) of GRBs. The decaying slope of the ATP shows time dilation when bright and dim bursts are compared, while the rising slope ha rdly changes. Both slopes of the ATP are deformed for weak bursts as compar ed to strong bursts. The interpretation of these effects is simple: a compl ex burst in which a number of independent pulses overlap in time appears to be intrinsically stronger than a simple burst. The BATSE sample of complex bursts then covers larger redshifts, where some cosmological factor causes the break in the peak brightness distribution. This break could correspond : to the peak in the star formation rate that was recently shown to occur a t a redshift of z similar to 1.5.