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We present a process that accounts for the steep-decline-and-plateau phase ofthe Swift-XRT light curves, vexing features of GRB phenomenology. This processis an integral part of the "supercritical pile" GRB model, proposed a few yearsago to provide an account for the conversion of the GRB kinetic energy intoradiation with a spectral peak at $E_{\rm pk} \sim m_ec^2$. We compute theevolution of the relativistic blast wave (RBW) Lorentz factor $\Gamma$ to showthat the radiation--reaction force due to the GRB emission can produce anabrupt, small ($\sim 25%$) decrease in $\Gamma$ at a radius which is smaller(depending on conditions) than the deceleration radius $R_D$. Because of thisreduction, the kinematic criticality criterion of the "supercritical pile" isno longer fulfilled. Transfer of the proton energy into electrons ceases, andthe GRB enters abruptly the afterglow phase at a luminosity smaller by $\simm_p/m_e$ than that of the prompt emission. If the radius at which thisslow-down occurs is significantly smaller than $R_D$, the RBW internal energycontinues to drive the RBW expansion at a constant (new) $\Gamma$, and itsX-ray luminosity remains constant until $R_D$ is reached, at which point itresumes its more conventional decay, thereby completing the "unexpected" XRTlight curve phase. If this transition occurs at $R \simeq R_D$, the steepdecline is followed by a flux decrease instead of a "plateau", consistent withthe conventional afterglow declines. Besides providing an account of thesepeculiarities, the model suggests that the afterglow phase may in fact beginbefore the RBW reaches $R \simeq R_D$, thus introducing novel insights into theGRB phenomenology.

The supercritical pile gamma-ray burst model: The GRB afterglow steep-decline-and-plateau phase