Discovery of a tight correlation among the prompt emission properties of long gamma-ray bursts

We report the discovery of a correlation among three prompt emission properties of gamma-ray bursts (GRBs). These are the isotropic peak luminosity L-iso, the peak energy (in nu L-nu) of the time-integrated prompt emission spectrum E-pk, and the 'high signal' time-scale T-0.45, previously used to characterize the variability behaviour of bursts. In the rest frame of the source, the discovered correlation reads L-iso alpha E-pk(1.62) T-0.45(-0.49). We find other strong correlations, but at the cost of increasing the number of variables, involving the variability and the isotropic energy of the prompt emission. With respect to the other tight correlations found in GRBs (i.e. between the collimation corrected energy E-gamma and E-pk, the so-called Ghirlanda correlation, and the phenomenological correlation among the isotropic emitted energy E-iso, E-pk and the jet break time t(break)), the newly found correlation does not require any information from the afterglow phase of the bursts, nor any model-dependent assumption. In the popular scenario in which we are receiving beamed radiation originating in a fireball pointing at us, the discovered correlation preserves its form in the comoving frame. This helps to explain the small scatter of the correlation and underlines the role of the local brightness (i.e. the brightness of the visible fraction of the fireball surface). This correlation has been found with a relatively small number of objects and it is hard to establish if any selection bias affects it. Its connection with the prompt local brightness is promising, but a solid physical understanding is still to be found. Despite all that, we find that some properties of the correlation, which we discuss, support its true existence, and this has important implications for the GRB physics. Furthermore, it is possible to use such correlation as an accurate redshift estimator, and, more importantly, its tightness will allow us to use it as a tool to constrain the cosmological parameters.