Binary neutron star (NS) mergers may result in remnants of supra-massive or even stable NS, which have been supported indirectly by observed X-ray plateau of some gamma-ray burst (GRB) afterglows. Recently, Xue et al. (2019) discovered an X-ray transient CDF-S XT2 that is powered by a magnetar from merger of double NS via X-ray plateau and following stepper phase. However, the decay slope after the plateau emission is slightly larger than the theoretical value of spin-down in electromagnetic (EM) dominated by losing its rotation energy. In this paper, we assume that the feature of X-ray emission is caused by a supra-massive magnetar central engine for surviving thousands of seconds to collapse into a black hole. Within this scenario, we present the comparisons of the X-ray plateau luminosity, break time, and the parameters of magnetar between CDF-S XT2 and other short GRBs with internal plateau samples. By adopting the collapse time to constrain the equation of state (EOS), we find that three EOSs (GM1, DD2, and DDME2) are consistent with the observational data. On the other hand, if the most released rotation energy of magnetar is dominated by GW radiation, we also constrain the upper limit of ellipticity of NS for given EOS, and its range is [0.32-1.3] 10⁻³. Its GW signal cannot be detected by Advanced LIGO or even for more sensitive Einstein Telescope in the future.

双中子星 (NS) 合并可能会产生超大质量甚至稳定 NS 的残余，这些残余已被观测到的一些伽马射线爆发 (GRB) 余辉的 X 射线平台间接支持。最近，薛等人。(2019) 发现了一个 X 射线瞬态 CDF-S XT2，它由磁星提供动力，该磁星通过 X 射线平台和后续步进阶段的双 NS 合并。然而，平台发射后的衰减斜率略大于电磁（EM）中以失去其旋转能量为主的自旋下降的理论值。在本文中，我们假设 X 射线发射的特征是由一个超大质量的磁星中央引擎在数千秒内坍缩成黑洞引起的。在这种情况下，我们展示了 X 射线平台光度、中断时间、CDF-S XT2与其他内部高原样本短伽玛暴之间的磁星参数。通过采用坍缩时间约束状态方程（EOS），我们发现三个EOS（GM1、DD2和DDME2）与观测数据一致。另一方面，如果磁星释放的最大旋转能量以引力波辐射为主，我们也限制了给定EOS的NS椭圆度上限，其范围为[0.32-1.3] 10^－3．它的引力波信号无法被先进的 LIGO 甚至未来更灵敏的爱因斯坦望远镜探测到。