Polarized signatures of adiabatically expanding hotspots in Sgr A*'s accretion flow

Abstract

We report 235 GHz linear and circular polarization (LP and CP) detections of Sgr A* at levels of $\sim 10\%$ and $\sim -1\%$, respectively, using ALMA. We describe the first full-Stokes modeling of an observed submillimeter flare with an adiabatically-expanding synchrotron hotspot using a polarized radiative transfer prescription. Augmented with a simple full-Stokes model for the quiescent emission, we jointly characterize properties of both the quiescent and variable components by simultaneously fitting all four Stokes parameter light curves. The hotspot has magnetic field strength $71$ G, radius $0.75$ Schwarzschild radii, and expands at speed $0.013$c assuming magnetic equipartition. The magnetic field's position angle projected in the plane-of-sky is $\approx 55^{\circ }$ East of North, which previous analyses reveal as the accretion flow's angular momentum axis and further supports Sgr A* hosting a magnetically-arrested disk. The magnetic field is oriented approximately perpendicular to the line of sight, which suggests repolarization as the cause of the high circular-to-linear polarization ratio observed at radio frequencies. We additionally recover several properties of the quiescent emission, consistent with previous analyses of the accretion flow, such as a rotation measure $\approx -4.22\times 10^5$ rad m${}^{-2}$. Our findings provide critical constraints for interpreting and mitigating the polarized variable emission in future Event Horizon Telescope images of Sgr A*.

arXiv comment13 pages, 6 figures, 3 tables. Accepted to Monthly Notices of the Royal Astronomical Society, comments welcome!

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Cited by in corpus (2)

• Polarimetry and Astrometry of NIR Flares as Event Horizon Scale, Dynamical Probes for the Mass of Sgr A*
• The internal Faraday screen of Sagittarius A*