黑洞X射线双星MAXI J1820 +070的光学偏振演化
Evolving optical polarisation of the black hole X-ray binary MAXI J1820+070
摘要：
目标。在X射线爆发期间，黑洞瞬态的光辐射增加了几个数量级。额外的光是来自于被加热的外盘的X射线，还是来自于内部的热吸积流，还是来自于喷流，目前还存在争议。光学偏振测量能够区分这些组件的相对贡献。方法.我们介绍了2018年3月至4月期间黑洞X射线双星MAXI J1820+070的BVR偏振测量结果。结果我们检测到较小的极化，$\sim$ 0.7%，但统计上显着，其中一部分起源于星际。根据星际偏振估计，源的固有偏振度在$\sim$ 0.3%和0.7%之间，偏振位置角在$\sim10°-30°$之间。我们发现，在MJD 58222（2018年4月14日）之后，极化增加。这种变化的数量级为0.1%，在R波段最为明显。源斯托克斯参数的变化与观测到的V带通量的下降和X射线谱的缓慢软化同时发生。在下降之前和之后的固有偏振的斯托克斯矢量是平行的，至少在V和R滤波器中是如此。结论.我们认为，增加极化是由于非极化分量的贡献减少，我们与热流或喷流排放。低极化可能是由于磁场的纠缠几何结构，或者是由于靠近黑洞的致密、电离和磁化介质中的法拉第旋转。偏振的光发射可能是由被照射的盘或其辐射在光学薄流出物中的散射产生的。
英文摘要：
Aims. The optical emission of black hole transients increases by several magnitudes during the X-ray outbursts. Whether the extra light arises from the X-ray heated outer disc, from the inner hot accretion flow, or from the jet is currently debated. Optical polarisation measurements are able to distinguish the relative contributions of these components. Methods. We present the results of BVR polarisation measurements of the black hole X-ray binary MAXI J1820+070 during the period of March-April 2018. Results. We detect small, $\sim$0.7%, but statistically significant polarisation, part of which is of interstellar origin. Depending on the interstellar polarisation estimate, the intrinsic polarisation degree of the source is between $\sim$0.3% and 0.7%, and the polarisation position angle is between $\sim10°-30°$. We show that the polarisation increases after MJD 58222 (2018 April 14). The change is of the order of 0.1% and is most pronounced in the R band. The change of the source Stokes parameters occurs simultaneously with the drop of the observed V-band flux and a slow softening of the X-ray spectrum. The Stokes vectors of intrinsic polarisation before and after the drop are parallel, at least in the V and R filters. Conclusions. We suggest that the increased polarisation is due to the decreasing contribution of the non-polarized component, which we associate with the the hot flow or jet emission. The low polarisation can result from the tangled geometry of the magnetic field or from the Faraday rotation in the dense, ionised, and magnetised medium close to the black hole. The polarized optical emission is likely produced by the irradiated disc or by scattering of its radiation in the optically thin outflow.
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