BRIGHTNESS-DEPENDENT PROPERTIES OF GAMMA-RAY BURSTS

Brightness-dependent correlations have been conclusively detected in t he average time profiles of gamma-ray bursts (GRBs). Determining the t ime constants of the stretched exponential slopes of the average peak- aligned time profile of GRBs as a function of the peak brightness, we find that the postpeak slope shows time dilation when comparing bright and dim bursts, while the prepeak slope hardly changes. Stronger burs ts are thus more symmetric than weaker bursts at a high confidence lev el. The very weakest bursts have a different shape (i.e., a different stretched exponential index) of their average time profile as compared to brighter bursts. This difference is too large to be explained by t rigger effects and Poisson noise. We interpret these correlations as b eing the result of an intrinsic positive correlation between brightnes s and complexity of GRBs. This interpretation is directly confirmed by simulations as well as by a morphological classification of bursts. T he fact that such a correlation can be observed should impose new cons traints on the distribution of GRBs over luminosity distance.