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

TL;DR

This paper investigates whether the X-ray transient XRT 201423 originated from a magnetar formed after a binary neutron star merger, using observational data to constrain the magnetar's properties and the source's distance.

Contribution

It provides the first detailed analysis testing the magnetar model for XRT 201423, deriving constraints on magnetic field, ellipticity, and distance based on X-ray and optical observations.

Findings

Data are consistent with a magnetar origin.

Magnetar surface magnetic field $< 7 imes 10^{14}$ G.

Distance upper limit depends on X-ray efficiency, e.g., $z < 0.55$ for $ ext{eta}_x=10^{-4}$.

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.