Unraveling Twisty Linear Polarization Morphologies in Black Hole Images

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.

26 pages, 20 figures, accepted for publication in ApJ

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Cited by in corpus (11)

• First M87 Event Horizon Telescope Results. IX. Detection of Near-horizon Circular Polarization
• Black Hole Polarimetry I: A Signature of Electromagnetic Energy Extraction
• Demonstrating Photon Ring Existence with Single-Baseline Polarimetry
• Near-Horizon Polarization as a Diagnostic of Black Hole Spacetime
• Bayesian Black Hole Photogrammetry
• Inferring Black Hole Spin from Interferometric Measurements of the First Photon Ring: A Geometric Approach
• Semi-analytic studies of accretion disk and magnetic field geometry in M87*
• Deep learning inference with the Event Horizon Telescope II. The Zingularity framework for Bayesian artificial neural networks
• KHARMA: Flexible, Portable Performance for GRMHD
• On the Universality of Energy Extraction from Black Hole Spacetimes
• Jet Archaeology and Forecasting: Image Variability and Magnetic Field Configuration