Magnetars—highly magnetized neutron stars—are thought to be the most likely progenitors for fast radio bursts (FRBs). Freely precessing magnetars are further invoked to explain the repeating FRBs. We report here on new high-cadence radio observations of the magnetar XTE J1810–197 recorded shortly after an X-ray outburst. We interpret the polarization variations of the magnetar radio emission as evidence for the magnetar undergoing free precession following the outburst while its magnetosphere slowly untwists. The observations of precession being damped on a timescale of months argue against the scenario of freely precessing magnetars as the origin of repeating FRBs. Using free-precession models based on relaxing ellipticity with a decay of the wobble angle, we find the magnetar ellipticity to be in good agreement with theoretical predictions from nuclear physics. Our precise measurement of the magnetar’s geometry can also further help in refining the modelling of X-ray light curves and constrain the star’s compactness.

磁星——高度磁化的中子星——被认为是快速射电爆发（FRB）最有可能的起源。进一步引用自由进动磁星来解释重复的快速射电暴。我们在此报告 X 射线爆发后不久记录的磁星 XTE J1810-197 的新高节奏射电观测结果。我们将磁星无线电发射的偏振变化解释为磁星在爆发后经历自由进动的证据，同时其磁层缓慢解开。对进动在数月时间尺度内衰减的观测结果与自由进动磁星作为重复快速射电暴起源的观点相矛盾。使用基于松弛椭圆率和摆动角衰减的自由进动模型，我们发现磁星椭圆率与核物理学的理论预测非常一致。我们对磁星几何形状的精确测量还可以进一步帮助完善 X 射线光变曲线的建模并限制恒星的致密性。