Circular polarization images of Sgr A* for different magnetic field geometries

TL;DR

This study models circular polarization images of Sgr A* using various magnetic field geometries and accretion parameters, constraining the magnetic field structure through comparison with observational data.

Contribution

It systematically analyzes CP images for six magnetic field configurations in a RIAF model, revealing how field geometry and black hole spin influence CP production and polarity.

Findings

Radial and parabolic fields produce polarity-invariant net CP.

Dipole and vertical fields yield polarity-sensitive net CP.

Comparison with ALMA data constrains Sgr A*'s magnetic field geometry.

Abstract

Sgr A* exhibits a persistent negative circular polarization (CP) at 230\,GHz, offering a powerful probe of the magnetic field geometry in its accretion flow. Using a stationary semi-analytic radiatively inefficient accretion flow (RIAF) model in Kerr spacetime with polarized radiative transfer, we systematically analyze CP images for six poloidal magnetic field configurations across varying black hole spins, inclinations, and field polarities. We find that CP production is dominated by Faraday conversion in radial, parabolic, quadrupole, and combined geometries, but by intrinsic emission in dipole and vertical fields. The radial and parabolic configurations produce the polarity-invariant net CP, while dipole and vertical fields yield the polarity-sensitive one. As the accretion disk is prograde with respect to the black hole spin, the CP production across all six field geometries is found to be lower at high spin case, while the situation is more complicated in the retrograde case. Moreover, the net CP observed from edge-on views $V_{\rm net} \approx 0$ except for the quadrupole geometry. Comparing with ALMA data, the reversed-field model is excluded at high inclinations and then the magnetic field geometry of Sgr A* is constrained.