Probing Accretion Turbulence in the Galactic Center with EHT Polarimetry

TL;DR

This paper investigates how interstellar scattering affects polarized images of Sagittarius A* and proposes a mitigation scheme to measure turbulence in black hole accretion flows using VLBI polarimetry.

Contribution

The authors develop a scattering mitigation method that enables accurate measurement of intrinsic image fluctuations related to accretion turbulence in the Galactic Center.

Findings

Scattering is non-birefringent for credible parameters.

The mitigation scheme effectively recovers intrinsic fluctuations.

Current and future experiments can measure the turbulence power spectrum.

Abstract

Magnetic fields grown by instabilities driven by differential rotation are believed to be essential to accretion onto black holes. These instabilities saturate in a turbulent state; therefore, the spatial and temporal variability in the horizon-resolving images of Sagittarius A* (Sgr A*) will be able to empirically assess this critical aspect of accretion theory. However, interstellar scattering blurs high-frequency radio images from the Galactic center and introduces spurious small-scale structures, complicating the interpretation of spatial fluctuations in the image. We explore the impact of interstellar scattering on the polarized images of Sgr A* and demonstrate that for credible physical parameters, the intervening scattering is non-birefringent. Therefore, we construct a scattering mitigation scheme that exploits horizon-resolving polarized millimeter/submillimeter VLBI observations to generate statistical measures of the intrinsic spatial fluctuations and therefore the underlying accretion flow turbulence. An optimal polarization basis is identified, corresponding to measurements of the fluctuations in magnetic field orientation in three dimensions. We validate our mitigation scheme using simulated data sets and find that current and future ground-based experiments will readily be able to accurately measure the image-fluctuation power spectrum.