Near-Horizon Polarization as a Diagnostic of Black Hole Spacetime

TL;DR

This paper identifies a unique near-horizon polarization pattern caused by frame dragging, which can serve as a robust diagnostic of black hole spin and spacetime geometry in high-resolution observations.

Contribution

It provides the first covariant analysis linking near-horizon polarization patterns directly to black hole spacetime properties, independent of plasma flow details.

Findings

Discovered a universal polarization pattern near black hole horizons.

Pattern remains consistent across different plasma flow structures.

Potential for future observations to measure black hole spin precisely.

Abstract

A key challenge in imaging supermassive black holes is disentangling gravitational effects from plasma physics in order to accurately determine spacetime properties, particularly black hole spin. In this Letter, we present a fully covariant and rigorous analysis of the synchrotron emission from accreting plasma in the equatorial plane in the stationary, axisymmetric, high-conductivity regime, and identify--for the first time--a distinctive near-horizon polarization pattern that remains robust across different flow structures. This pattern arises from strong frame dragging near the event horizon, which induces a degeneracy among plasma flow and magnetic field configurations, yielding a polarization signature determined solely by the spacetime geometry and the observer's inclination. The near-horizon polarization thus offers a clean and precise probe of black hole spin and other fundamental parameters. If future space-based millimeter VLBI observations can resolve synchrotron emission originating within approximately 1$\%$ of the event horizon radius in M87* or Sgr A*, this universal polarization pattern may become observable.