Alma polarimetry of Sgr A∗: Probing the accretion flow from the event horizon to the bondi radius

Millimeter polarimetry of Sgr A∗ probes the linearly polarized emission region on a scale of ~ 10 Schwarzschild radii (R_S) as well as the dense, magnetized accretion ow on scales out to the Bondi radius (~ 10⁵R_S) through Faraday rotation. We present here multi-epoch ALMA Band 6 (230 GHz) polarimetry of Sgr A∗. The results confirm a mean rotation measure, RM ≃ -5 × 10⁵rad m^-2, consistent with measurements over the past 20 years and support an interpretation of the RM as originating from a radiatively inefficient accretion flow (RIAF) with M ≃ 10^-8M_oy^-1. Variability is observed for the first time in the RM on time scales that range from hours to months. The long-term variations may be the result of changes in the line of sight properties in a turbulent accretion ow. Short-term variations in the apparent RM are not necessarily the result of Faraday rotation and may be the result of complex emission and propagatation effects close to the black hole, some of which have been predicted in numerical modeling. We also confirm the detection of circular polarization at a mean value of -1.1±0.2%. It is variable in amplitude on time scales from hours to months but the handedness remains unchanged from that observed in past centimeter- and millimeter-wavelength detections. These results provide critical constraints for the analysis and interpretation of Event Horizon Telescope data of Sgr A∗, M87, and similar sources.