Accounting for the GRB Afterglow Steep-decline-and-plateau Phase
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 dissipation of the GRB kinetic energy and its conversion into radiation with spectrum which peaks at an energy $E_{\rm pk} \sim m_ec^2$, {\em at the observer's frame}, in agreement with observations. As discussed earlier, the emission of the prompt GRB phase at $E_{\rm pk} \simeq 0.2-1$ MeV in this model is due not to synchrotron (as is conventionally accepted) but to the bulk Comptonization of synchrotron photons which have scattered upstream of the GRB relativistic blast wave (RBW) and are re-intercepted by it. We compute the evolution of the RBW Lorentz factor $\Gamma$ to show that the radiation--reaction force due to this process can produce an abrupt (i.e. over a distance much shorter than the RBW deceleration radius $R_D$), small ($\sim 25\%$) decrease in $\Gamma$ at a radius which is also smaller (depending on conditions) than $R_D$. Because of the reduction in $\Gamma$, the kinematic criticality criterion of the ``supercritical pile" is no longer fulfilled. Transfer of the proton energy into electrons ceases, $E_{\rm pk}$ shifts to lower energies and the GRB enters abruptly the afterglow phase at a luminosity smaller by $\sim m_p/m_e$ than that of the prompt emission. Furthermore, because 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) $\Gamma$. During this stage, its X-ray luminosity remains constant until the value of $R_D$ is reached, at which point it resumes its more conventional decay, thereby completing the ``unexpected" XRT light curve phase. Besides providing an account of the ``peculiar" GRB afterglow light curves, this model suggests that the afterglow phase may in fact begin before the RBW reaches its deceleration radius, thus introducing novel insights into the GRB phenomenology.
- Publication:
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AAS/High Energy Astrophysics Division #13
- Pub Date:
- April 2013
- Bibcode:
- 2013HEAD...1312404K