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 $B_p<7\times 10^{14}\mathrm{G}$ ($B_p<4.9\times 10^{14}\mathrm{G}$) 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.

8 pages, 1 figure, accepted for publication in ApJ Letters

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