Black hole X-ray binaries exhibit different spectral and timing properties in different accretion states. The X-ray outburst of a recently discovered and extraordinarily bright source, Swift J1727.8–1613, has enabled the first investigation of how the X-ray polarization properties of a source evolve with spectral state. The 2–8 keV polarization degree was previously measured by the Imaging X-ray Polarimetry Explorer (IXPE) to be ≈4% in the hard and hard intermediate states. Here we present new IXPE results taken in the soft state, with the X-ray flux dominated by the thermal accretion disk emission. We find that the polarization degree has dropped dramatically to ≲1%. This result indicates that the measured X-ray polarization is largely sensitive to the accretion state and the polarization fraction is significantly higher in the hard state when the X-ray emission is dominated by upscattered radiation in the X-ray corona. The combined polarization measurements in the soft and hard states disfavor a very high or low inclination of the system.

中文翻译：

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Swift J1727.8–1613 在软光谱状态下的 X 射线偏振急剧下降

黑洞X射线双星在不同的吸积状态下表现出不同的光谱和计时特性。最近发现的异常明亮的光源 Swift J1727.8-1613 的 X 射线爆发，首次研究了光源的 X 射线偏振特性如何随光谱状态演化。先前通过成像 X 射线偏振测量探测器 (IXPE) 测量，在硬态和硬中间态下，2-8 keV 偏振度约为 4%。在这里，我们展示了在软状态下获得的新 IXPE 结果，其中 X 射线通量主要由热吸积盘发射决定。我们发现偏振度急剧下降至≲1%。该结果表明，测量的 X 射线偏振对吸积态非常敏感，并且当 X 射线发射以 X 射线日冕中的上散射辐射为主时，硬态下的偏振分数显着更高。软态和硬态下的组合偏振测量不利于系统的极高或极低的倾角。