The role of afterglow break-times as gamma-ray burst jet angle indicators

The early X-ray light curve of gamma-ray bursts (GRBs) is complex, and shows a typical steep-flat-steep behaviour. The time T-a at which the flat (plateau) part ends may bear some important physical information, especially if it plays the same role of the so called jet break-time t(jet). To this aim, stimulated by the recent analysis of Willingale et al., we have assembled a sample of GRBs of known redshifts, spectral parameters of the prompt emission, and T-a. By using T-a as a jet angle indicator, and then estimating the collimation-corrected prompt energetics, we find a correlation between the latter quantity and the peak energy of the prompt emission. However, this correlation has a large dispersion, similar to the dispersion of the Amati correlation and it is not parallel to the Ghirlanda correlation. Furthermore, we show that the correlation itself results mainly from the dependence of the jet opening angle on the isotropic prompt energy, with the time T-a playing no role, contrary to what we find for the jet break-time t(jet). We also find that for the bursts in our sample T-a weakly correlates with E-gamma,E-iso of the prompt emission, but that this correlation disappears when considering all bursts of known redshift and T-a. There is no correlation between T-a and the isotropic energy of the plateau phase.