We present a comprehensive sample of X-ray observations of 41 \gamma -ray burst ( GRB ) afterglows , as well as jet opening angles , \theta _ { j } for a subset with measured jet breaks .
We show that there is a significant dispersion in the X-ray fluxes , and hence isotropic X-ray luminosities ( L _ { X, { iso } } ) , normalized to t = 10 hr .
However , there is a strong correlation between L _ { X, { iso } } and the beaming fractions , f _ { b } \equiv [ 1 - { cos } ( \theta _ { j } ) ] .
As a result , the true X-ray luminosity of GRB afterglows , L _ { X } = f _ { b } L _ { X, { iso } } , is approximately constant , with a dispersion of only a factor of two .
Since \epsilon _ { e } E _ { b } \propto L _ { X } , the strong clustering of L _ { X } directly implies that the adiabatic blastwave kinetic energy in the afterglow phase , E _ { b } , is tightly clustered .
The narrow distribution of L _ { X } also suggests that p \approx 2 , that inverse Compton emission does not in general dominate the observed X-ray luminosity , and that radiative losses at t < 10 hr are relatively small .
Thus , despite the large diversity in the observed properties of GRBs and their afterglows the energy imparted by the GRB central engine to the relativistic ejecta is approximately constant .