AI 中文总结
本研究通过IXPE、NICER和NuSTAR的联合观测，分析天鹅座X-1低硬态吸积dip的光谱与偏振，发现2-8 keV波段偏振度和角度基本不变，支持X射线偏振源于延展扁球形吸积盘冕的结论。
AI 中文摘要
天鹅座X-1是研究恒星质量黑洞X射线双星吸积机制的基础基准源。传统X射线时变和光谱学已描绘出其双峰态跃迁特征，但要精确约束吸积等离子体的几何构型，还需补充其他观测工具。本研究利用IXPE、NICER和NuSTAR的同时观测数据，对天鹅座X-1低硬态下流量下降事件（即“dips”）开展联合光谱-偏振分析。宽带光谱模型显示，这些dips中与能量相关的流量下降，最适合用部分覆盖吸收模型描述，且对软X射线波段影响显著。该吸收与伴星恒星风或外吸积盘在 superior conjunction 期间产生的致密冷团块遮蔽效应一致。值得注意的是，偏振分析表明，尽管流量大幅下降，但2-8 keV波段的偏振度和偏振角在dip与非dip时段基本保持一致。这种高吸收时段偏振特征的稳定性，说明被遮蔽的热吸积盘辐射对总偏振的贡献不大。最终，这些结果有力支持X射线偏振源于延展的、可能呈扁球形的吸积盘冕的观点，其大尺度结构不受局域团块影响。
英文摘要
Cygnus X-1 serves as a foundational benchmark for studying accretion mechanisms in stellar-mass black hole X-ray binaries. While traditional X-ray timing and spectroscopy have mapped its bimodal state transitions, constraining the exact geometric configuration of its accreting plasma requires additional tools. In this letter, we present a joint spectro-polarimetric analysis of Cygnus X-1 during flux-decrease events, or ``dips'', observed in its low hard state using simultaneous data from IXPE, NICER, and NuSTAR. The broadband spectral model reveals that the energy-dependent flux reduction during these dips is best described by a partial-covering absorption model, heavily impacting the softer energy bands. This absorption is consistent with obscuration by dense, cold clumps originating from the companion's stellar wind or the outer accretion disk during superior conjunction. Notably, our polarimetric analysis demonstrates that despite significant flux drops, the polarization degree and polarization angle in the 2-8 keV band remain largely consistent between dip and off-dip intervals. The stability of the polarization signature during these highly absorbed periods indicates that the obscured thermal disk emission does not significantly contribute to the total polarization. Ultimately, these results strongly support a scenario where the X-ray polarization originates from an extended, likely oblate, accretion-disk corona whose large scale renders its overall geometry unaffected by localized clumping.
