Least-square fit to the X-ray lightcurves of four selected GRB afterglows observed by Swift/XRT obtained through our Eqs. (8) and (9). The blue points and red lines represent the measured (isotropic) luminosity (in the 0.3–100 keV energy range) and best fits, respectively. The starting time T₀ was determined for each afterglow based on the end of the spectral transition from the previous steep-decay phase, as discussed in the text. Best-fit parameter values for each individual lightcurve are reported in Table 1.