磁层双折射打破旋转矢量模型并使X射线脉冲星退偏振
Magnetospheric birefringence breaks the rotating-vector model and depolarises X-ray pulsars
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中文总结 AI 辅助
该研究发现吸积中子星磁层的双折射效应，会使X射线脉冲星的偏振角分布拓宽、线偏振度降低，打破旋转矢量模型预期，可解释观测到的低偏振分数与反常角摆动。
中文摘要 AI 辅助
长期以来，人们一直认为吸积X射线脉冲星在回旋共振以下会呈现出大的线偏振度。在标准图像中，这源于两个事实：强磁化中子星环境中的辐射不透明度强烈依赖于偏振模式，且辐射来自致密的极区。因此，出射辐射应是局域偏振的，且几乎相干叠加，偏振角会像旋转矢量模型所描述的那样追踪投影磁场。然而，近期的X射线偏振观测显示，偏振分数适中，且偏振角摆动常常偏离这一简单图像。在此，我们表明，吸积中子星的磁层传播可自然产生这两种效应。利用广义相对论光线追踪和斯托克斯参数传输，我们发现从致密极区发射的光子会通过两种方式获得依赖于光线的偏振角偏移：第一，近乎沿磁场方向的轨迹会穿过QED双折射区域内的局域非绝热区域；第二，与等离子体负载吸积流相交的光子会在等离子体本征模式之间积累额外的相位偏移。这两种效应都会拓宽光线束的偏振角分布，降低热点积分的线偏振度，同时产生偏离旋转矢量模型预期的系统偏差。因此，磁层传播应被视为吸积X射线脉冲星光偏振传输问题的一部分，或有助于解释观测到的低偏振分数和反常角摆动。
英文摘要
Accreting X-ray pulsars have long been expected to show large linear polarisation below the cyclotron resonance. In the standard picture, this follows from two facts: radiative opacities in the strongly magnetised neutron star environment depend strongly on polarisation mode, and the emission comes from compact polar regions. The emerging radiation should therefore be locally polarised and add almost coherently, with a polarisation angle tracing the projected magnetic field as in the rotating-vector model. Recent X-ray polarimetric observations, instead, show modest polarisation fractions and position angle swings that often depart from this simple picture. Here we show that propagation through the magnetosphere of an accreting neutron star can naturally produce both effects. Using general-relativistic ray tracing and Stokes-parameter transport, we find that photons emitted from compact polar regions acquire ray-dependent polarisation angle offsets in two ways. First, nearly field-aligned trajectories can cross local non-adiabatic pockets inside the QED birefringent region. Second, photons that intersect the plasma-loaded accretion flow accumulate additional phase shifts between plasma normal modes. Both effects broaden the distribution of polarisation angles across the ray bundle and reduce the hotspot-integrated linear polarisation, while also producing systematic deviations from rotating-vector-model expectations. Magnetospheric propagation should therefore be treated as part of the polarimetric transfer problem in accreting X-ray pulsars, and may help explain the observed low polarisation fractions and anomalous angle swings.
