We investigate wheter there is any correlation between the X–ray afterglow luminosity and the prompt emission properties of a carefully selected sub-sample of bright Swift long Gamma–Ray Bursts ( GRBs ) nearly complete in redshift ( \sim 90 \% ) .
Being free of selection effects ( except flux limit ) , this sample provides the possibility to compare the rest frame physical properties of GRB prompt and afterglow emission in an unbiased way .
The afterglow X–ray luminosities are computed at four different rest frame times ( 5 min , 1 hr , 11 hr and 24 hr after trigger ) and compared with the prompt emission isotropic energy E _ { iso } , the isotropic peak luminosity L _ { iso } and the rest frame peak energy E _ { peak } .
We find that the rest frame afterglow X–ray luminosity do correlate with these prompt emission quantities , but the significance of each correlation decreases over time .
This result is in agreement with the idea that the GRB X-ray light curve can be described as the result of a combination of different components whose relative contribution and weight change with time , with the prompt and afterglow emission dominating at early and late time , respectively .
In particular , we found evidence that the plateau and the shallow decay phase often observed in GRB X–ray light curves are powered by activity from the central engine .
The existence of the L _ { X } - E _ { iso } correlation at late times ( t _ { rf } \geq 11 hr ) suggests a similar radiative efficiency among different bursts with on average about 6 % of the total kinetic energy powering the prompt emission .