Dependence of linear polarization of radiation in accretion disks on the spin of central black hole

TL;DR

This paper investigates how the linear polarization of radiation from accretion disks depends on the black hole's spin, showing that Kerr black holes produce higher polarization than Schwarzschild ones due to magnetic field effects.

Contribution

It demonstrates the dependence of polarization degree on black hole spin by analyzing magnetic field effects and the radius of the stable orbit in accretion disks.

Findings

Kerr black holes exhibit higher polarization than Schwarzschild black holes.

Magnetic field strength and Faraday rotation significantly influence polarization levels.

Most observed polarized objects are likely Kerr black holes.

Abstract

We suppose that linear optical polarization is due to multiple scattering in optically thick magnetized accretion disk around central black hole. The polarization degree is very sensitive to the spin of black hole - for Kerr rotating hole the polarization is higher than for Schwarzschild non-rotating one if both holes have the same luminosities and masses. The reason of this effect is that the radius of the first stable orbit for non-rotating hole is equal to three gravitational radiuses, and for fast rotating Kerr hole is approximately 6 times lesser. Magnetic field, decreasing from first stable orbits, is much larger in the region of escaping of optical radiation for the case of Schwarzschild hole than for Kerr one. Large magnetic field gives rise to large depolarization of radiation due to Faraday rotation effect. This explains the mentioned result. It seems that the ensemble of objects with observed polarization mostly consists of Kerr black holes.