用气体云探测SgrA周围的吸积流 *：G2的延迟近心通过
Using gas clouds to probe the accretion flow around SgrA*: G2's delayed pericenter passage
摘要：
我们研究了最近在银河系中心发现的气体云G1和G2的动力学演化。根据早期的研究表明，这两个云是一个更大的气体流光的一部分，我们将它们的轨道合并成一个单一的轨迹。由于气体云受到来自背景气体的拖曳力，因此这个轨迹并不完全是开普勒轨道。通过假设G1和G2云遵循这一轨迹，我们拟合了它们所经历的拖曳力，从而提取了距离黑洞数千个史瓦西半径的吸积流的信息。这个半径范围对于黑洞吸积理论是很重要的，但目前还没有观测到。在本文中，我们扩展了我们以前的工作占径向力由于可能的流入或流出的背景气体。这种径向力驱动轨道平面中的进动，允许稍微更好地拟合G1和G2数据。相对于开普勒轨道拟合的估计，这种岁差使G2的近心通过延迟了4 - 5个月;如果证明有可能通过观测确定近心时间，这就可以立即测试G1和G2是否是较大气体流光的气体云的一部分。如果G2确实是一个气体云，那么它最接近地球的时间可能是在2014年夏末，当时许多监测G2预期的近心点通过的观测活动已经结束。我们将讨论这如何影响G2观测的解释。
英文摘要：
We study the dynamical evolution of the putative gas clouds G1 and G2 recently discovered in the Galactic center. Following earlier studies suggesting that these two clouds are part of a larger gas streamer, we combine their orbits into a single trajectory. Since the gas clouds experience a drag force from background gas, this trajectory is not exactly Keplerian. By assuming the G1 and G2 clouds trace this trajectory, we fit for the drag force they experience and thus extract information about the accretion flow at a distance of thousands of Schwarzschild radii from the black hole. This range of radii is important for theories of black hole accretion, but is currently unconstrained by observations. In this paper we extend our previous work by accounting for radial forces due to possible inflow or outflow of the background gas. Such radial forces drive precession in the orbital plane, allowing a slightly better fit to the G1 and G2 data. This precession delays the pericenter passage of G2 by 4-5 months relative to estimates derived from a Keplerian orbital fit; if it proves possible to identify the pericenter time observationally, this enables an immediate test of whether G1 and G2 are gas clouds part of a larger gas streamer. If G2 is indeed a gas cloud, its closest approach likely occurred in late summer 2014, after many of the observing campaigns monitoring G2's anticipated pericenter passage ended. We discuss how this affects interpretation of the G2 observations.
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