On the intrinsic nature of the updated luminosity time correlation in the X-ray afterglows of GRBs

Gamma-ray bursts (GRBs) observed up to redshifts z > 9.3 are fascinating objects to study due to their still unexplained relativistic outburst mechanisms and a possible use to test cosmological models. Our analysis of all GRB afterglows with known redshifts and definite plateau (100 GRBs) reveals not only that the luminosity L* _X (T_a ) - break time T* _a correlation, called hereafter LT, (Dainotti et al. 2010a) is confirmed with higher value of the Spearman correlation coefficient for the new updated sample, but also reveals its intrinsic nature throughout the analysis of the Efron & Petrosian (1992) test. The above mentioned test is performed to check if there is redshift evolution in both the luminosity and time. This test shows that the correlation still holds probing that its nature is intrinsic and it is not due to selection biases. The novelty of this approach is that the Efron & Petrosian method has been applied for the first time for a two parameter correlation that involves not only luminosities, but also time. Notwithstanding the intrinsic nature of the correlation, the correction of the observables for the effect of redshift evolution does not lead to a significantly tighter correlation and thus to a better redshift estimator. Therefore, the usage of the L* _a correlation is limited, at least with the present data analysis, to constrain physical models of plateau emission. With an enlarged data sample in the future the aim will be to make the luminosity time correlation a useful redshift estimator.