利用X射线光谱学和偏振测量学约束掩食Atoll源4U 1624-49的几何结构
Constraining the geometry of the dipping atoll 4U 1624-49 with X-ray spectroscopy and polarimetry
AI总结：
本研究通过IXPE、NuSTAR和NICER同步观测掩食中子星X射线双星4U 1624-49，发现其偏振度高达2.7%±0.9%，结合光谱拟合与高倾角（约60°）揭示了其几何结构，并指出康普顿化辐射是偏振信号的主要来源。
摘要：
我们展示了通过IXPE、NuSTAR和NICER对掩食中子星X射线双星4U 1624-49进行同步X射线观测所获得的光谱偏振结果。该源是迄今为止用IXPE观测到的偏振程度最高的Atoll源，在非掩食相期间2-8 keV能段内的偏振度为2.7% ± 0.9%，并有随能量增加的边缘性证据。与其他Atoll源相比偏振度更高，可以通过该系统的高倾角（i ≈ 60°）来解释。光谱可以很好地用软热辐射、康普顿化分量加上软光子在吸积盘上的反射的组合来描述。在掩食期间，高度电离吸收体的氢柱密度增加，而电离态降低。康普顿化辐射似乎是偏振信号的主要贡献者，加上额外的反射光子，即使它们在总通量中的比例不高，也有显著贡献。
英文摘要：
We present the spectro-polarimetric results obtained from simultaneous X-ray observations with IXPE, NuSTAR and NICER of the dipping neutron star X-ray binary 4U 1624-49. This source is the most polarized Atoll source so far observed with IXPE, with a polarization degree of 2.7% $\pm$ 0.9% in the 2-8 keV band during the non-dip phase and marginal evidence of an increasing trend with energy. The higher polarization degree compared to other Atolls can be explained by the high inclination of the system ($i \approx 60$°). The spectra are well described by the combination of a soft thermal emission, a Comptonized component, plus reflection of soft photons off the accretion disk. During the dips, the hydrogen column density of the highly-ionized absorber increases while the ionization state decreases. The Comptonized radiation seems to be the dominant contribution to the polarized signal, with additional reflected photons which significantly contribute even if their fraction in the total flux is not high.
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