Study of possible systematics in the L*X - Ta* correlation of Gamma Ray
  Bursts
  Gamma Ray Bursts (GRBs) are the most energetic sources in the universe and
among the farthest known astrophysical sources. These features make them
appealing candidates as standard candles for cosmological applications so that
studying the physical mechanisms for the origin of the emission and
correlations among their observable properties is an interesting task. We
consider here the luminosity L*X - break time Ta* (hereafter LT) correlation
and investigate whether there are systematics induced by selection effects or
redshift dependent calibra- tion. We perform this analysis both for the full
sample of 77 GRBs with known redshift and for the subsample of GRBs having
canonical X-ray light curves, hereafter called U0095 sample. We do not find any
systematic bias thus con- firming the existence of physical GRB subclasses
revealed by tight correlations of their afterglow properties. Furthermore, we
study the possibility of applying the LT correlation as a redshift estimator
both for the full distribution and for the canonical lightcurves. The large
uncertainties and the non negligible intrin- sic scatter make the results not
so encouraging, but there are nevertheless some hints motivating a further
analysis with an increased U0095 sample.
