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arXiv 2401.16239astro-ph.HE
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X射线双星中弱磁化中子星周围边界层的极化

The polarization of the boundary layer around weakly magnetized neutron stars in X-ray binaries

R. Farinelli, A. Waghmare, L. Ducci, A. Santangelo

AI总结:

本研究针对X射线双星偏振观测数据难以区分组分贡献的问题,构建了软态低质量X射线双星中弱磁化中子星边界层的偏振计算模型,通过辐射转移求解与相对论坐标转换得到偏振特征,为IXPE等观测数据解读提供理论支持并可消除多组分分析的简并性。

摘要:

承载致密天体的X射线双星自成像X射线偏振探测卫星(Imaging X-ray Polarimetry Explorer,IXPE)发射以来,因其在2-8 keV能段的高亮度,一直是该卫星的主要观测目标之一。迄今开展的光谱偏振分析已被证明对约束这类系统的吸积几何结构具有重要意义。然而,现有数据的统计量不足以明确区分各单一组分对观测到的净偏振信号的贡献。在本工作中,我们旨在提出一个模型,用于计算软态下低质量X射线双星中弱磁化中子星周围边界层的偏振度和偏振角。研究的主要动机是为IXPE或未来X射线偏振测量卫星的观测数据解读提供有力的理论支撑。我们通过将边界层建模为致密天体周围的赤道环带,并在局部将其近似为平行平面散射大气,求解了汤姆孙极限下偏振辐射对应的辐射转移方程,从而得到了相关结果。随后,我们利用此前为X射线脉冲星开发的数值方法,将偏振量从共动坐标系转换到观测者坐标系。对于软态下这类系统边界层的典型光深和电子温度值,偏振度低于0.5%;而由于快速自转产生的狭义和广义相对论效应,偏振角相对于中子星自转轴的偏转小于5°,偏转量随自转频率降低而逐渐减小。所得的物理量可用于在开展多组分光谱偏振测量时消除简并。

英文摘要:

X-ray binaries hosting a compact object have been among the main targets of the Imaging X-ray Polarimetry Explorer (IXPE) since its launch, due to their high brightness in the 2-8 keV energy band. The spectropolarimetric analysis performed so far has proved to be of great importance in providing constraints on the accretion geometry of these systems. However, the data statistics is not enough to unambiguously disentangle the contribution of the single components to the net observed polarimetric signal. In this work, we aim to present a model for computing the polarization degree and polarization angle of the boundary layer around weakly magnetized neutron stars in low-mass X-ray binaries in the soft state. The main motivation is to provide strong theoretical support to data interpretation of observations performed by IXPE or future satellites for X-ray polarimetry. The results were obtained by modeling the boundary layer as an equatorial belt around the compact object and locally approximating it as a plane-parallel scattering atmosphere, for which the associated radiative transfer equation for polarized radiation in the Thomson limit was solved. The polarimetric quantities were then transformed from the comoving frame to the observer frame using the numerical methods formerly developed for X-ray pulsars. For typical values of the optical depth and electron temperature of the boundary layer of these systems in a soft state, the polarization degree was less then 0.5\%, while the polarization angle was rotated by $\protect \la 5^{\circ}$ with respect to the neutron star spin axis due to special and general relativistic effects for fast rotation, the amount progressively decreasing for lower spin frequencies. The derived quantities can be used to remove degeneracy when multicomponent spectropolarimetry is performed.

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