Black Hole Magnetic Fields and Their Imprint on Circular Polarization Images

TL;DR

This paper investigates how magnetic field structures around black holes influence circular polarization images, using GRMHD simulations to identify observable signatures and effects like sign flipping of photon rings.

Contribution

It introduces a detailed analysis of magnetic field geometry effects on circular polarization images and highlights novel light bending phenomena affecting these signals.

Findings

Circular polarization sign stability relates to magnetic field properties.

Magnetic field geometry imprints distinct features on polarization images.

Light bending causes sign flipping in photon rings, revealing magnetic field handedness.

Abstract

The circular polarization of black hole accretion flows can encode properties of the underlying magnetic field structure. Using general relativistic magnetohydrodynamics (GRMHD) simulations, we study the imprint of magnetic field geometry on circular polarization images potentially observable by the Event Horizon Telescope (EHT). We decompose images into the different mechanisms that generate circular polarization in these models, which are sensitive to both the line of sight direction and twist of the magnetic field. In these models, a stable sign of the circular polarization over time, as observed for several sources, can be attributed to a stability of these properties. We illustrate how different aspects of a generic helical magnetic field geometry become imprinted on a circular polarization image. We also identify novel effects of light bending that affect the circular polarization image on event horizon scales. One consequence is the sign flipping of successive photon rings in face-on systems, which if observable and uncorrupted by Faraday rotation, can directly encode the handedness of the approaching magnetic field.