重力合作组织在2018年7月22日的耀斑期间检测到一个近红外热点在Sgr A * 周围移动。他们用围绕超大质量黑洞（BH）的半径为$\simeq7.5\,r_{\rm g}$的圆形开普勒轨道拟合了热点在天空中形成的部分环路，其中$r_{\rm g}$是引力半径。然而，由于热点在短时间内穿过环路，因此热点跟踪某些流体元素运动的模型往往会产生比观察到的环路更小的最佳拟合轨迹。这是真实的圆形开普勒轨道，即使当BH自旋占，并沿RIAF流线运动。一条边缘有界的测地线也有同样的问题;此外，不清楚一个遵循测地线的物体的原点是什么。观察到的热点运动更可能是一种模式运动。圆周运动与$r\simeq12.5\,r_{\rm g}$和超开普勒速度$\simeq0.8\,c$是一个很好的适合。这种运动必须是模式运动，因为它不能用物理力来解释。如果磁场足够强，则图案速度与磁流体动力学扰动相容。黑洞上方平面上半径为$\sim20\, r_{\rm g}$的环形运动也是一个很好的选择;在这种情况下，热点可能是由旋进外流与周围盘相互作用引起的。由于我们所有的拟合都有相对较大的半径，我们不能用这些观测来约束BH自旋。
英文摘要：
The Gravity Collaboration detected a near-infrared hotspot moving around Sgr A* during the 2018 July 22 flare. They fitted the partial loop the hotspot made on the sky with a circular Keplerian orbit of radius $\simeq7.5\,r_{\rm g}$ around the supermassive black hole (BH), where $r_{\rm g}$ is the gravitational radius. However, because the hotspot traversed the loop in a short time, models in which the hotspot tracks the motion of some fluid element tend to produce a best-fit trajectory smaller than the observed loop. This is true for a circular Keplerian orbit, even when BH spin is accounted for, and for motion along a RIAF streamline. A marginally bound geodesic suffers from the same problem; in addition, it is not clear what the origin of an object following the geodesic would be. The observed hotspot motion is more likely a pattern motion. Circular motion with $r\simeq12.5\,r_{\rm g}$ and a super-Keplerian speed $\simeq0.8\,c$ is a good fit. Such motion must be pattern motion because it cannot be explained by physical forces. The pattern speed is compatible with magnetohydrodynamic perturbations, provided that the magnetic field is sufficiently strong. Circular pattern motion of radius $\sim20\, r_{\rm g}$ on a plane above the BH is an equally good alternative; in this case, the hotspot may be caused by a precessing outflow interacting with a surrounding disk. As all our fits have relatively large radii, we cannot constrain the BH spin using these observations.
Nonthermal hotspot orbiting the supermassive black hole Sgr A*
Instituto de Astronomía, Universidad Nacional Autónoma de México（墨西哥国立自治大学天文研究所）；Instituto Nacional de Astrofísica, Óptica y Electrónica（墨西哥国家天文学、光学和电子学研究所）；Tsung-Dao Lee Institute, Shanghai Jiao Tong University（上海交通大学李政道研究所）；School of Physics and Astronomy, Shanghai Jiao Tong University（上海交通大学物理与天文学院）
Bayesian Evidence for Inspiraling Hotspot Motion in the Galactic Center
Wake Forest University（维克森林大学）；Instituto de Astrofísica de Andalucía–CSIC（安达卢西亚天体物理研究所-西班牙国家研究委员会）；Black Hole Initiative at Harvard University（哈佛大学黑洞倡议项目）
Constraints on Buchdahl-Inspired Gravity from Future Pulsar Timing near Sgr A*
University of Chinese Academy of Sciences（中国科学院大学）；Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences（中国科学院大学杭州高等研究院）；Zhejiang University of Technology（浙江工业大学）；United Center for Gravitational Wave Physics (UCGWP), Zhejiang University of Technology（浙江工业大学引力波物理联合中心）；Institute of Theoretical Physics, Chinese Academy of Sciences (CAS)（中国科学院理论物理研究所）；Peking University（北京大学）
Fundamental Physics with Pulsars around Sagittarius A$^\star$
Kavli Institute for Astronomy and Astrophysics, Peking University（北京大学 Kavli天体物理与天文研究所）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；Department of Astronomy, School of Physics, Peking University（北京大学物理学院天文学系）
