The impact of Faraday effects on polarized black hole images of Sagittarius A

We study model images and polarization maps of Sagittarius A* at 230 GHz. We post-process general relativistic magnetohydrodynamic simulations and perform a fully relativistic radiative transfer calculation of the emitted synchrotron radiation to obtain polarized images for a range of mass accretion rates and electron temperatures. At low-accretion rates, the polarization map traces the underlying toroidal magnetic field geometry. At high-accretion rates, we find that Faraday rotation internal to the emission region can depolarize and scramble the map. We measure the net linear polarization fraction and find that high-accretion rate 'jet-disc' models are heavily depolarized and are therefore disfavoured. We show how Event Horizon Telescope measurements of the polarized 'correlation length' over the image provide amodel independent upper limit on the strength of these Faraday effects, and constrain plasma properties like the electron temperature and magnetic field strength.