中子星极区捕获的中子星海洋表面重力波
Surface gravity wave on a neutron star ocean trapped around a magnetic pole
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中文总结 AI 辅助
本研究提出中子星磁极区捕获的表面重力波可解释部分中子星X射线双星的低频准周期振荡，其频率特性与星震学g模式类似。
中文摘要 AI 辅助
热中子星的外 crust 最外层预计存在由重元素构成的流体“海洋”。与地球海洋类似，中子星海洋也存在表面重力波。在恒星磁极附近，强磁压会参与海洋的流体静力学平衡，可能使海洋形成凹陷；该凹陷可让表面重力波被捕获在磁极附近，形成本征模。由于科里奥利力较弱，且海洋深度梯度导致本征模存在，该模式的频率远低于恒星表面的动力学频率。我们在局部β平面近似下求解表面重力波方程，得到离散本征模的频谱。结果显示，不存在轴对称模式；随着对应本征函数的节点数增加，模式频率降低并渐近趋于零，这与星震学中的g模式类似。对包含中子星的X射线双星的观测表明，部分系统呈现频率为1-10³mHz的低频准周期振荡（QPO）。我们研究了本文考虑的本征模是否可解释该低频QPO频谱，结果表明，中子星自转周期小于10s的系统中，部分QPO可能与模型一致；而当自转周期大于10s时，本征模频率过低，无法解释观测到的QPO。
英文摘要
A warm neutron star is expected to have a fluid "ocean" of heavy elements at its outermost part of the outer crust. As is on the terrestrial ocean, the neutron star ocean also has surface gravity waves. Around a magnetic pole of a star, the ocean may have a dip due to the strong magnetic pressure coming into play in the hydrostatic balance of the ocean. The dip enables the surface gravity wave to be trapped around the magnetic pole to form eigenmodes. The frequency of the mode is much lower than the dynamical frequency at the stellar surface, owing to the weak Coriolis force and the gradient in the ocean's depth that makes the eigenmodes present. We solve the equation of surface gravity waves in the local $β$-plane approximation and obtain the spectrum of discrete eigenmodes. We see that there are no axisymmetric modes and that the mode frequency decreases and asymptotes to zero as the number of nodes of the corresponding eigenfunction increases. This is reminiscent of the g-modes in the context of asteroseismology. Observations of X-ray binaries containing neutron stars reveal that some of the systems exhibit low-frequency quasi-periodic oscillations (QPOs) whose frequency is $1-10^3$mHz. We investigate whether the eigenmodes considered here may explain the low-frequency QPO spectrum. It is suggested that some of the QPOs in the system whose neutron star spins at the period less than $10$s may be consistent with the model. As far as the spin period is larger than $10$s, the eigenmode frequencies are too low to explain the observed QPOs.
