论文标题
围绕sgr a*:轨道还是图案运动的热点的起源?
The origin of hotspots around Sgr A*: Orbital or pattern motion?
论文作者
论文摘要
重力合作检测到2018年7月22日耀斑期间的近红外热点围绕SGR A*移动。他们安装了热点在天空上制成的部分环,上面是半径$ \ simeq7.5 \,r _ {\ rm g} $的圆形开普勒轨道,围绕超质量的黑洞(BH),其中$ r _ {\ rm g} $是坟墓radius。但是,由于热点在短时间内遍历了循环,因此热点轨迹轨迹的运动趋向于产生小于观察到的环的最佳拟合轨迹。对于圆形开普勒轨道,即使是BH旋转,也是沿RIAF流线运动的情况,也是如此。略有绑定的大地测量遭受了相同的问题;此外,尚不清楚遵循测量的对象的来源是什么。观察到的热点运动更可能是模式运动。带有$ r \ simeq12.5 \,r _ {\ rm g} $的循环运动和超级凯普利安速度$ \ simeq0.8 \,c $非常适合。这种运动必须是模式运动,因为它不能用物理力来解释。如果磁场足够强,则模式速度与磁流失动力动力扰动兼容。半径$ \ sim20 \,r _ {\ rm g} $的圆形模式运动在BH上方的平面上是同样好的替代方案;在这种情况下,热点可能是由与周围磁盘相互作用的前进流出引起的。由于我们所有的拟合都具有相对较大的半径,因此我们无法使用这些观察结果约束BH旋转。
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.