Variability of Polarization and Intensity as Probes of the Accretion Flow of Sgr A*

我们银河系的中心有一个超大质量黑洞，称为Sgr A*。黑洞的质量是太阳的400万倍。它最初是通过其附近的热气体发出的无线电辐射被发现的，黑洞区域的亮度以小时为单位变化。射电天文学家在过去的十年里观测到了这种变化。 这个项目的目标是利用现有的数据来研究黑洞的性质。 研究人员将研究黑洞附近亮度变化的物质流动。一个重要的目标是测量黑洞的旋转方向，并将其与整个银河系的旋转方向进行比较。该分析将通过对过去10年来黑洞的变化进行建模来测量旋转轴。 研究人员在西北大学的主图书馆发起了两个关于射电天文学的公开展览。本次展览将集中展示射电天文学在我们日常生活中的影响、遍布地球仪的射电望远镜的多样性，以及详细射电图像的艺术方面。研究人员重点关注与Sgr A* 变化相关的三个主题。一个是解决来自Sgr A* 的光学厚可变发射的磁场取向。这个角度与吸积流的自旋轴和大尺度喷流特征有何关系？为了解决这个问题，将进行绝热膨胀同步加速器热点的全斯托克斯分析。第二是模拟同时观测从美国国家科学基金会的甚大阵列（VLA）和阿塔卡马大型毫米/亚毫米阵列（阿尔马），以表征的固有可变性的Sgr A* 的应用绝热膨胀模型。利用Sgr A* 的光学、红外和射电光变曲线，研究人员将确定多次耀斑的亚毫米/射电频谱和发射变异性。第三，将使用过去十年中使用VLA采集的VLA存档数据，以说明长于每小时时间尺度的变异性的性质。这项研究将确定基本的可变性统计的动态时间尺度长达十年，以确定热点的物理参数是否随时间而变化，告知历史的吸积流。这个奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

The center of our Milky Way Galaxy hosts a supermassive black hole, known as Sgr A*. The black hole is 4 million times the mass of the Sun. It was first identified by radio emission from hot gas orbiting near it. The brightness of the black hole regions changes on hourly time scales. Radio astronomers have observed this variability for the last ten years. The goal of this project is to use available data to look at the properties of the black hole. The investigator will study the flow of material near the black hole where its brightness changes. An important goal is measuring the direction the black hole rotates and comparing it with the rotation direction of the entire Milky Way. The analysis will measure the rotation axis by modeling the changes in the black hole over the past 10 years. The investigator has initiated two public exhibits on radio astronomy at Northwestern's main library. This exhibit will focus on the impact of radio astronomy in our daily lives, the diversity of radio telescopes throughout the globe, and artistic aspects of detailed radio images.The investigator focuses on three topics related to the variability of Sgr A*. One is to address the magnetic field orientation of optically thick variable emission from Sgr A*. How does this angle relate to the spin axis of the accretion flow and the large-scale jet-like feature. To address this issue, full-Stokes analyses of adiabatically-expanding synchrotron hotspot will be carried out. Second is to model simultaneous observations from NSF’s Very Large Array (VLA) and Atacama Large Millimeter/Submillimeter Array (ALMA), to characterize the intrinsic variability of Sgr A* by applying the adiabatic expansion model. Using optical, infrared and radio light curves of Sgr A* the investigator will determine the submm/radio spectrum and emission variability across multiple flares. Third, archival VLA data taken with the VLA over the last ten years will be used to address the nature of variability on longer-than-hourly time scales. This study will determine the dynamics of the underlying variability statistics on timescales up to a decade, in order to determine if the physical parameters of hotspots have changed with time, informing the history of the accretion flow.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.