On the Binary-Neutron-Star Post-Merger Magnetar Origin of XRT 210423

Abstract: XRT 201423 is an X-ray transient with a nearly flat plateau lasting 4.1 ks followed by a steep decay. This feature indicates that it might come from a magnetar formed through a binary neutron star merger, similar to CDF-S XT2 and as predicted as a type of electromagnetic counterpart of binary neutron star mergers. We test the compliance of the data with this model and use the observed duration and flux of the X-ray signal as well as upper limits of optical emission to pose constraints on the parameters of the underlying putative magnetar. Both the free-zone and trapped-zone geometric configurations are considered. We find that the data are generally consistent with such a model. The surface dipolar magnetic field and the ellipticity of the magnetar should satisfy $B_p < 7\times 10^{14}{\rm G}$ ($B_p < 4.9 \times 10^{14}{\rm G}$) and $\epsilon < 1.5 \times 10^{-3}$ ($\epsilon < 1.1 \times 10^{-3}$) under free zone (trapped zone) configurations, respectively. An upper limit on the distance (e.g. $z < 0.55$ with $\eta_x = 10^{-4}$ or $z < 3.5$ with $\eta_x = 10^{-2}$) can be derived from the X-ray data which depends on the X-ray dissipation efficiency $\eta_x$ of the spin-down luminosity. The non-detection of an optical counterpart places a conservative lower limit on the distance of the source, i.e. $z > 0.045$ regardless of the geometric configuration.