Constraining Radiatively Inefficient Accretion Flows with Polarization

TL;DR

This paper demonstrates that polarization measurements of Sgr A* can constrain the physical properties of Radiatively Inefficient Accretion Flows by analyzing the effects of generalized Faraday rotation caused by elliptical plasma normal modes.

Contribution

It introduces the concept that high electron temperatures in RIAFs lead to elliptical plasma modes, affecting polarization and enabling new constraints on accretion flow models.

Findings

Polarization undergoes generalized Faraday rotation without depolarization.

Polarization properties reflect plasma normal modes influenced by temperature and magnetic fields.

Measurements can constrain temperature, density, and magnetic profiles of RIAFs.

Abstract

The low-luminosity black hole Sgr A* provides a testbed for models of Radiatively Inefficient Accretion Flows (RIAFs). Recent sub-millimeter linear polarization measurements of Sgr A* have provided evidence that the electrons in the accretion flow are relativistic over a large range of radii. Here, we show that these high temperatures result in elliptical plasma normal modes. Thus, polarized millimeter and sub-millimeter radiation emitted within RIAFs will undergo generalized Faraday rotation, a cyclic conversion between linear and circular polarization. This effect will not depolarize the radiation even if the rotation measure is extremely high. Rather, the beam will take on the linear and circular polarization properties of the plasma normal modes. As a result, polarization measurements of Sgr A* in this frequency regime will constrain the temperature, density and magnetic profiles of RIAF models.