自旋-盘错位驱动X射线脉冲星的周期性吸积与脉冲轮廓不对称性
Spin-disc misalignment drives periodic accretion and pulse profile asymmetry in X-ray pulsars
AI总结：
该研究通过数值模拟发现，自旋-盘错位的纯几何效应会驱动X射线脉冲星的周期性吸积，导致脉冲轮廓不对称，该机制也可能适用于ULX脉冲星。
AI中文摘要：
我们研究了由吸积盘供给的X射线脉冲星中的磁层吸积，假设中子星自转轴与吸积盘平面并不垂直。我们聚焦于X射线脉冲星，这类脉冲星中几何薄吸积盘被截断在远离恒星表面的位置，吸积流的引导部分在与盘-磁层边界耦合后，预计会被大尺度偶极磁场引导。通过对从内盘边缘到中子星表面的等离子体运动进行数值模拟，我们表明，即使吸积盘提供的质量供给是稳定的，盘法线与恒星自转轴之间的有限倾角也会导致磁极上的质量吸积率发生周期性调制。这种纯几何效应的产生是因为恒星自转改变了磁层相对于吸积盘的取向，从而产生与相位相关的磁场线质量加载。调制的振幅和形状由系统几何以及恒星自转周期与流体通过磁层的时间之比决定。由此产生的变率会影响中子星表面附近发射区的结构和光度，并导致X射线脉冲轮廓不对称。即使发射区没有固有不对称性，该机制也会打破稳态吸积预期的时间反演对称性，自然促成观测到的脉冲轮廓不对称性和相位分辨光谱特征。该效应可能也与ULX脉冲星相关，在这类脉冲星中，固有吸积率调制可帮助在几何聚束存在时保持强脉冲信号。
英文摘要：
We investigate magnetospheric accretion in disc-fed X-ray pulsars assuming that the neutron star spin axis is not perpendicular to the disc plane. We focus on X-ray pulsars, where a geometrically thin disc is truncated far from the stellar surface the channelled part of the accretion flow is expected to be guided by the large-scale dipolar magnetic field after coupling to it near the disc-magnetosphere boundary. Using numerical simulations of plasma motion from the inner disc edge to the neutron star surface, we show that a finite inclination between the disc normal and the stellar spin axis leads to periodic modulation of the mass accretion rate onto the magnetic poles even for a steady mass supply through the disc. This purely geometrical effect arises because stellar rotation changes the orientation of the magnetosphere relative to the disc, producing phase-dependent mass loading of magnetic field lines. The amplitude and shape of the modulation are determined by the system geometry and by the ratio of the stellar spin period to the flow time through the magnetosphere. The resulting variability affects the structure and luminosity of emitting regions near the neutron star surface and leads to asymmetric X-ray pulse profiles. Even without intrinsic asymmetries of the emission regions, this mechanism breaks the time-reversal symmetry expected for stationary accretion and naturally contributes to the observed asymmetry of pulse profiles and phase-resolved spectral features. The effect may also be relevant for ULX pulsars, where intrinsic accretion rate modulation can help preserve strong pulsations in the presence of geometric beaming.
