The Polarized Image of a Synchrotron Emitting Ring of Gas Orbiting a Black Hole

TL;DR

This paper develops an analytic model to interpret polarized synchrotron images of black hole accretion disks, successfully reproducing observed polarization patterns and providing insights into magnetic field configurations near black holes.

Contribution

It introduces a simple, analytic model of polarized emission from a black hole accretion disk that aligns with complex GRMHD simulation results and observations.

Findings

Model reproduces polarimetric image morphology of M87*

Predicts polarization evolution in flaring regions like Sgr A*

Consistent with observed EVPA patterns and rotation directions

Abstract

Synchrotron radiation from hot gas near a black hole results in a polarized image. The image polarization is determined by effects including the orientation of the magnetic field in the emitting region, relativistic motion of the gas, strong gravitational lensing by the black hole, and parallel transport in the curved spacetime. We explore these effects using a simple model of an axisymmetric, equatorial accretion disk around a Schwarzschild black hole. By using an approximate expression for the null geodesics derived by Beloborodov (2002) and conservation of the Walker-Penrose constant, we provide analytic estimates for the image polarization. We test this model using currently favored general relativistic magnetohydrodynamic simulations of M87*, using ring parameters given by the simulations. For a subset of these with modest Faraday effects, we show that the ring model broadly reproduces the polarimetric image morphology. Our model also predicts the polarization evolution for compact flaring regions, such as those observed from Sgr A* with GRAVITY. With suitably chosen parameters, our simple model can reproduce the EVPA pattern and relative polarized intensity in Event Horizon Telescope images of M87*. Under the physically motivated assumption that the magnetic field trails the fluid velocity, this comparison is consistent with the clockwise rotation inferred from total intensity images.