发表机构
Lomonosov Moscow State University; Sternberg Astronomical Institute; INAF-IAPS（莫斯科国立大学; 斯登伯格天文研究所; 意大利国家天体物理研究所-空间天文研究所）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究利用 IXPE 和 NICER 数据，分析黑洞双星 4U 1630-47 在中间态的 X 射线偏振，发现偏振度在 2-8 keV 能量范围内恒定，归因于平展康普顿云中的康普顿化，并据此确定光学深度。
AI 中文摘要
IXPE 任务使得对吸积双星中 X 射线偏振的测量成为可能。X 射线偏振度对双星轨道倾角以及发射介质光学深度 $\tau_0$ 的依赖，是 X 射线天文学中新的重要工具。我们以黑洞 X 射线双星 4U~1630--47 作为银河系中一个神秘源的代表进行了分析。我们研究了 IXPE 和 NICER 观测到的 4U~1630--47 爆发事件的能谱和偏振数据。我们仔细考察了 IXPE 观测中 4U~1630--47 处于单一光谱态（中间态）的时间区间，发现该时间段内偏振度在 2--8 keV 能段内随光子能量没有显著变化（$P\sim$ 6%）。我们将 $P(E)$ 的这种能量无关行为解释为软光子在该平展康普顿云中的康普顿化（上散射）的结果。我们认为，该 X 射线源的宽带能谱可以由一个由康普顿化成分和铁线（{\it 高斯}）成分组成的物理模型来重现。使用该模型的光谱分析表明，在中间态期间，4U~1630--47 的光谱具有光子指数 $\Gamma\sim 2.5$ 和电子温度 $kT_e\sim ~9 ~{\rm keV}$ 的特征。事实上，利用倾角 $69\pm1^\circ$ 并通过迭代方法求解平展康普顿云的完整辐射转移方程时，运用关系式 $(P,\cos(i),\tau_0)$，这种方法可以轻松得到 4U~1630--47 中康普顿云的半光学深度 $\tau_0\sim 1.8$。这种方法还伴随着偏振度与光子能量无关这一事实，我们至少在 2--8 keV 能段对 4U~1630--47 的观测中证明了这一点。
英文摘要
The IXPE mission enabled measurements of X-ray polarization in accreting binaries. The dependence of the X-ray polarization degree on the binary inclination and the optical depth of the emitting medium $τ_0$ are the new important tools in X-ray Astronomy. We analyzed the black hole X-ray binary 4U~1630--47 as an example of one of the enigmatic sources in our Galaxy. We consider spectral and polarimetric observations of outburst episodes from 4U~1630--47 observed with the IXPE and the NICER. We carefully investigated a time interval of the IXPE observations during which 4U~1630--47 was in a single spectral state (the intermediate state) and found that the polarization degree did not change significantly during this time interval with the photon energy in the 2--8 keV range ($P\sim$ 6\%). We interpreted this energy independence behavior in $P(E)$ as a result of the Comptonization (up-scattering) of soft photons in the flat Compton cloud. We argued that the X-ray source broadband energy spectra can be reproduced by a physical model composed of the Comptonized component and iron-line ({\it Gaussian}) component. Spectral analysis using this model shows that during the intermediate state the 4U~1630--47 spectrum was characterized by a photon index $Γ\sim 2.5$ and the electron temperature $kT_e\sim ~9 ~{\rm keV}$. In fact, this approach allows one to easily find the half-optical depth $τ_0\sim 1.8$ of the Compton cloud in 4U~1630--47 applying the inclination $69\pm1^\circ$ and using the relation $(P,\cos(i),τ_0)$ when solving the full radiative transfer equation for a flat Compton cloud by an iterative method. This approach is also accompanied by the fact that the polarization degree is independent of the photon energy, which we demonstrate observationally, at least for 4U~1630--47 in the 2--8 keV range.
Comments17 pages, 13 figures
