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

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 feature. We find that the photon ring is typically a factor of similar or equal to 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.

Automated summary

Through simulations of accreting black holes, it is found that subtracting linear polarization can enhance the photon ring feature, leading to precise measurements of black hole mass and spin.