A Complexity‐Brightness Correlation in Gamma‐Ray Bursts

We observe strong correlations between the temporal properties of gamma raybursts (GRBs) and their apparent peak brightness. The strongest effect (with asignificance level of 10^{-6}) is the difference between the brightnessdistributions of simple bursts (dominated by a single smooth pulse) and complexbursts (consisting of overlapping pulses). The latter has a break at a peakflux of 1.5 ph/cm^2/s, while the distribution of simple bursts is smooth downto the BATSE threshold. We also observe brightness dependent variations in theshape of the average peak aligned time profile (ATP) of GRBs. The decayingslope of the ATP shows time dilation when comparing bright and dim bursts whilethe rising slope hardly changes. Both slopes of the ATP are deformed for weakbursts as compared to strong bursts. The interpretation of these effects issimple: a complex burst where a number of independent pulses overlap in timeappears intrinsically stronger than a simple burst. Then the BATSE sample ofcomplex bursts covers larger redshifts where some cosmological factor causesthe break in the peak brightness distribution. This break could correspond tothe peak in the star formation rate that was recently shown to occur at aredshift of z~1.5.Comment: 13 pages; 11 figures; replaced with the published versio