Circular Polarization of Simulated Images of Black Holes

TL;DR

This study compares simulated black hole images with observed circular polarization data to constrain models of Sgr A* and M87*, revealing preferences for certain magnetic field orientations and accretion flow rotations.

Contribution

It introduces a comprehensive library of models varying key parameters and compares them to unresolved CP measurements, refining understanding of black hole environments.

Findings

ALMA data exclude all edge-on models for Sgr A*

MAD models with magnetic dipole away from observer fit the data

Most MAD models show clockwise accretion flow rotation

Abstract

Models of the resolved Event Horizon Telescope (EHT) sources Sgr A* and M87* are constrained by observations at multiple wavelengths, resolutions, polarizations, and time cadences. In this paper we compare unresolved circular polarization (CP) measurements to a library of models, where each model is characterized by a distribution of CP over time. In the library we vary the spin of the black hole, the magnetic field strength at the horizon (i.e. both SANE and MAD models), the observer inclination, a parameter for the maximum ion-electron temperature ratio assuming a thermal plasma, and the direction of the magnetic field dipole moment. We find that ALMA observations of Sgr A* are inconsistent with all edge-on ($i = 90^\circ$) models. Restricting attention to the magnetically arrested disk (MAD) models favored by earlier EHT studies of Sgr A*, we find that only models with magnetic dipole moment pointing away from the observer are consistent with ALMA data. We also note that in 26 of the 27 passing MAD models the accretion flow rotates clockwise on the sky. We provide a table of the mean and standard deviation of the CP distributions for all model parameters along with their trends.