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

TL;DR

This study models the polarized emission from a hotspot near Sgr A* using ALMA data, revealing magnetic field properties and dynamics that support the magnetically-arrested disk model and inform future Event Horizon Telescope observations.

Contribution

First full-Stokes modeling of a submillimeter flare in Sgr A* with an adiabatically-expanding hotspot, providing detailed magnetic and geometric insights.

Findings

Hotspot magnetic field strength of 71 G

Hotspot radius of 0.75 Schwarzschild radii

Magnetic field orientation supports magnetically-arrested disk model

Abstract

We report 235 GHz linear and circular polarization (LP and CP) detections of Sgr A* at levels of $\sim10\%$ 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 $\approx55^\circ$ 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 $\approx-4.22\times10^{5}$ 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*.