人马座A*视界尺度图像中出人意料的不对称性缺失
The Unexpected Lack of Asymmetry in the Horizon-Scale Image of Sagittarius A*
AI总结：
本研究提出傅里叶域量化黑洞图像亮度不对称性的方法，发现人马座A*图像不对称性低于M87，结合协变半解析模型得出其吸积流速度或黑洞自旋存在特殊约束的结论。
摘要：
事件视界望远镜（EHT）拍摄到的两个超大质量黑洞的环状图像，为研究视界尺度下的吸积流物理提供了有力工具。具体而言，由于光子在发射位置会经历多普勒增亮效应，图像中的亮度不对称性携带着内吸积流角速度分布以及观测者倾角的信息。本文提出了一种在傅里叶域量化黑洞图像亮度不对称性的方法，该方法可直接从干涉测量数据中进行测量。我们将此方法应用于当前的EHT数据，发现人马座A*（Sgr A*）的图像不对称程度异常低，甚至低于M87的推断值。随后我们利用协变半解析模型，对Sgr A*内吸积流的倾角和速度分布施加了约束。研究发现，如果等离子体速度遵循开普勒分布，显著亮度不对称性的缺失会迫使观测者倾角处于极不合理的小值范围（6-10°）。另一种可能是，如果等离子体速度显著低于开普勒速度且黑洞自转速度不快，则可以适配更大的倾角角度。
英文摘要：
The ring-like images of the two supermassive black holes captured by the Event Horizon Telescope (EHT) provide powerful probes of the physics of accretion flows at horizon scales. Specifically, the brightness asymmetry in the images carries information about the angular velocity profile of the inner accretion flow and the inclination of the observer, owing to the Doppler boosts photons experience at their site of emission. In this paper, we develop a method for quantifying the brightness asymmetry of black-hole images in the Fourier domain, which can be measured directly from interferometric data. We apply this method to current EHT data and find that the image of Sagittarius A* (Sgr A*) has an unusually low degree of asymmetry that is even lower than that inferred for M87. We then use a covariant semi-analytic model to obtain constraints on the inclinations and velocity profiles of the inner accretion flow for Sgr A*. We find that the lack of significant brightness asymmetry forces the observer inclination to uncomfortably small values ($6-10^\circ$), if the plasma velocity follows Keplerian profiles. Alternatively, larger inclination angles can be accommodated if the plasma velocities are significantly sub-Keplerian and the black hole is not spinning rapidly.
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