The Detection Prospects of the Polarizations in the Plateau Phase of the Gamma-Ray Burst Afterglow by eXTP

Abstract Approximately 20%–50% of the gamma-ray burst (GRB) X-ray afterglows exhibit shallow decay features. Two popular energy-injection models have been proposed to interpret such observational phenomena, the relativistic wind bubble (RWB) model with a Poynting-flux injection and the structured ejecta (SE) model with a dynamical energy injection. Polarization predictions of the two models have been investigated and can be used as a test of the two models. However, the impacts of the parameters on the model predictions were not studied, and the comparisons with the detection ability of the forthcoming mission, enhanced X-ray Timing and Polarimetry (eXTP), has not been discussed. We considered the above issues and found that influences of the model parameters on the predicted polarizations of the two models are very limited. To perform a feasible polarization detection during the plateau phase, a priority target-of-opportunity (ToO) response is required. The detection probability of the GRB plateau phase is about 1/3 for one pointing under a priority ToO. The polarization detection probability would depend on the ratio between the Poynting-flux injection to the dynamical energy injection, which is unclear currently. The predicted flux density and polarization degree (PD) of the RWB model could be well above the threshold flux and minimal detectable PD of the Polarimetry Focusing Array (PFA) on board eXTP, while the predicted PDs of the SE model would be difficult to be detected by eXTP/PFA. Therefore, a detection of a significant polarization signal during the GRB plateau phase would prefer the RWB model, and the injected energy would be in the form of the Poynting flux, while a no detection of the polarized signal would indicate a dynamical energy injection of the SE model.