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 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 $B_p 0.045$ regardless of the geometric configuration.