Radio emission from magnetars provides a unique probe of the relativistic, magnetized plasma within the near-field environment of these ultra-magnetic neutron stars. The transmitted waves can undergo birefringent and dispersive propagation effects that result in frequency-dependent conversions of linear to circularly polarized radiation and vice versa, thus necessitating classification when relating the measured polarization to the intrinsic properties of neutron star and fast radio burst emission sites. We report the detection of such behaviour in 0.7–4 GHz observations of the P = 5.54 s radio magnetar XTE J1810−197 following its 2018 outburst. The phenomenon is restricted to a narrow range of pulse phase centred around the magnetic meridian. Its temporal evolution is closely coupled to large-scale variations in magnetic topology that originate from either plastic motion of an active region on the magnetar surface or free precession of the neutron star crust. Our model of the effect deviates from simple theoretical expectations for radio waves propagating through a magnetized plasma. Birefringent self-coupling between the transmitted wave modes, line-of-sight variations in the magnetic field direction and differences in particle charge or energy distributions above the magnetic pole are explored as possible explanations. We discuss potential links between the immediate magneto-ionic environments of magnetars and those of fast radio burst progenitors.

磁星的无线电发射为这些超磁性中子星的近场环境中的相对论磁化等离子体提供了独特的探针。发射波可能会经历双折射和色散传播效应，导致线偏振辐射与圆偏振辐射的频率相关转换，反之亦然，因此在将测量到的偏振与中子星和快速射电爆发发射点的固有属性相关时需要进行分类。我们报告了在 2018 年爆发后P  = 5.54 s 射电磁星 XTE J1810−197 爆发后在 0.7–4 GHz 观测中检测到的这种行为。这种现象仅限于以磁子午线为中心的窄脉冲相位范围。它的时间演化与磁拓扑的大规模变化密切相关，这些变化源于磁星表面活动区域的塑性运动或中子星地壳的自由进动。我们的效应模型偏离了无线电波通过磁化等离子体传播的简单理论预期。探索了传输波模式之间的双折射自耦合、磁场方向的视线变化以及磁极上方粒子电荷或能量分布的差异作为可能的解释。我们讨论了磁星的直接磁离子环境与快速射电爆发祖细胞之间的潜在联系。