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

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.