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Magnetic fields are believed to drive accretion and relativistic jets inblack hole accretion systems, but the magnetic-field structure that controlsthese phenomena remains uncertain. We perform general relativistic (GR)polarized radiative transfer of time-dependent three-dimensional GRmagnetohydrodynamical (MHD) simulations to model thermal synchrotron emissionfrom the Galactic Center source Sagittarius A$^\ast$ (Sgr A*). We compare ourresults to new polarimetry measurements by the Event Horizon Telescope (EHT)and show how polarization in the visibility (Fourier) domain distinguishes andconstrains accretion flow models with different magnetic field structures.These include models with small-scale fields in disks driven by themagnetorotational instability (MRI) as well as models with large-scale orderedfields in magnetically-arrested disks (MAD). We also consider differentelectron temperature and jet mass-loading prescriptions that control thebrightness of the disk, funnel-wall jet, and Blandford-Znajek-driven funneljet. Our comparisons between the simulations and observations favor models withordered magnetic fields near the black hole event horizon in Sgr A*, althoughboth disk- and jet-dominated emission can satisfactorily explain most of thecurrent EHT data. We show that stronger model constraints should be possiblewith upcoming circular polarization and higher frequency ($349~{\rm GHz}$)measurements.

Probing the magnetic field structure in Sgr A* on Black Hole Horizon Scales with Polarized Radiative Transfer Simulations