Unraveling Twisty Linear Polarization Morphologies in Black Hole Images

TL;DR

This study uses GRMHD simulations and a simple ring model to link the twisty linear polarization patterns in black hole images to magnetic field structures and black hole spin, with implications for understanding black hole environments.

Contribution

It demonstrates that the twisty polarization morphology is primarily determined by magnetic field geometry and its dependence on black hole spin, with minimal influence from Doppler boosting and lensing.

Findings

Twisty polarization patterns are linked to magnetic field structure.

Spin affects polarization morphology via magnetic field changes due to frame dragging.

Faraday rotation's impact is minor in strong magnetic field models.

Abstract

We investigate general relativistic magnetohydrodynamic simulations (GRMHD) to determine the physical origin of the twisty patterns of linear polarization seen in spatially resolved black hole images and explain their morphological dependence on black hole spin. By characterising the observed emission with a simple analytic ring model, we find that the twisty morphology is determined by the magnetic field structure in the emitting region. Moreover, the dependence of this twisty pattern on spin can be attributed to changes in the magnetic field geometry that occur due to the frame dragging. By studying an analytic ring model, we find that the roles of Doppler boosting and lensing are subdominant. Faraday rotation may cause a systematic shift in the linear polarization pattern, but we find that its impact is subdominant for models with strong magnetic fields and modest ion-to-electron temperature ratios. Models with weaker magnetic fields are much more strongly affected by Faraday rotation and have more complicated emission geometries than can be captured by a ring model. However, these models are currently disfavoured by the recent EHT observations of M87*. Our results suggest that linear polarization maps can provide a probe of the underlying magnetic field structure around a black hole, which may then be usable to indirectly infer black hole spins. The generality of these results should be tested with alternative codes, initial conditions, and plasma physics prescriptions.