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

TL;DR

This paper investigates how Faraday effects influence polarized images of Sagittarius A* at 230 GHz, revealing how accretion rates affect polarization and providing constraints on plasma properties through EHT measurements.

Contribution

It introduces a detailed relativistic radiative transfer analysis of polarized emission from Sagittarius A* across different accretion rates, highlighting Faraday effects' impact on polarization.

Findings

High accretion rates cause depolarization due to Faraday rotation.

Jet-disc models are disfavored because of heavy depolarization.

EHT measurements constrain plasma properties and Faraday effects.

Abstract

We study model images and polarization maps of Sagittarius A* at 230 GHz. We post-process GRMHD 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 a model-independent upper limit on the strength of these Faraday effects, and constrain plasma properties like the electron temperature and magnetic field strength.