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

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 (BNS) merger, similar to CDF-S XT2 and as predicted as a type of electromagnetic counterpart of BNS 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 Bp < 7 × 1014G (Bp < 4.9 × 1014G) and epsilon < 1.5 × 10−3 (epsilon < 1.1 × 10−3) under free zone (trapped zone) configurations, respectively. An upper limit on the distance (e.g., z < 0.55 with ηx = 10−4 or z < 3.5 with ηx = 10−2) can be derived from the X-ray data, which depends on the X-ray dissipation efficiency ηx of the spin-down luminosity. The nondetection 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.