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

TL;DR

This paper introduces a model explaining the steep decline and plateau phases in GRB afterglow light curves by analyzing the relativistic blast wave dynamics and radiation reaction effects, offering new insights into GRB phenomenology.

Contribution

It presents a novel explanation for the steep-decline-and-plateau phases in GRB afterglows based on the supercritical pile model and blast wave Lorentz factor evolution.

Findings

Radiation reaction causes a small decrease in Lorentz factor, ending prompt emission.

The afterglow can start before the blast wave reaches the deceleration radius.

The model accounts for both plateau and steep decline phases in X-ray light curves.

Abstract

We present a process that accounts for the steep-decline-and-plateau phase of the Swift-XRT light curves, vexing features of GRB phenomenology. This process is an integral part of the "supercritical pile" GRB model, proposed a few years ago to provide an account for the conversion of the GRB kinetic energy into radiation with a spectral peak at $E_{\rm pk} \sim m_ec^2$. We compute the evolution of the relativistic blast wave (RBW) Lorentz factor $\Gamma$ to show that the radiation--reaction force due to the GRB emission can produce an abrupt, small ($\sim 25%$) decrease in $\Gamma$ at a radius which is smaller (depending on conditions) than the deceleration radius $R_D$. Because of this reduction, the kinematic criticality criterion of the "supercritical pile" is no longer fulfilled. Transfer of the proton energy into electrons ceases, and the GRB enters abruptly the afterglow phase at a luminosity smaller by $\sim m_p/m_e$ than that of the prompt emission. If the radius at which this slow-down occurs is significantly smaller than $R_D$, the RBW internal energy continues to drive the RBW expansion at a constant (new) $\Gamma$, and its X-ray luminosity remains constant until $R_D$ is reached, at which point it resumes its more conventional decay, thereby completing the "unexpected" XRT light curve phase. If this transition occurs at $R \simeq R_D$, the steep decline is followed by a flux decrease instead of a "plateau", consistent with the conventional afterglow declines. Besides providing an account of these peculiarities, the model suggests that the afterglow phase may in fact begin before the RBW reaches $R \simeq R_D$, thus introducing novel insights into the GRB phenomenology.