Constraining Radiatively Inefficient Accretion Flows with Polarization

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Abstract
The low-luminosity black hole Sgr A* provides a test bed for models of radiatively inefficient accretion flows (RIAFs). Recent submillimeter 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 submillimeter 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.
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