Relative depolarization of the black hole photon ring in GRMHD models of Sgr A* and M87*

TL;DR

This study demonstrates that subtracting linear polarization from total intensity images in GRMHD simulations enhances black hole photon ring features, aiding in measurements of black hole properties and tests of general relativity.

Contribution

It introduces a polarization subtraction method that improves photon ring visibility in simulated black hole images, facilitating better parameter estimation.

Findings

Photon ring is about twice less polarized than surrounding regions.

Subtracting residual polarization enhances photon ring sharpness.

Method is effective for models of Sgr A* and M87*, aiding black hole studies.

Abstract

Using general relativistic magnetohydrodynamic simulations of accreting black holes, we show that a suitable subtraction of the linear polarization per pixel from total intensity images can enhance the photon ring features. We find that the photon ring is typically a factor of $\simeq 2$ less polarized than the rest of the image. This is due to a combination of plasma and general relativistic effects, as well as magnetic turbulence. When there are no other persistently depolarized image features, adding the subtracted residuals over time results in a sharp image of the photon ring. We show that the method works well for sample, viable GRMHD models of Sgr A* and M87*, where measurements of the photon ring properties would provide new measurements of black hole mass and spin, and potentially allow for tests of the "no-hair" theorem of general relativity.