The Black Hole Mass and Magnetic Field Correlation in AGN: Testing by Optical Polarimetry

TL;DR

This study uses optical polarimetry to test the relationship between black hole magnetic fields and mass in AGN, finding that the correlation does not hold for NGC 4258, thus questioning previous assumptions.

Contribution

It introduces a method to estimate black hole magnetic fields via polarization spectra, testing the B-M relation in AGN using optical polarimetry.

Findings

The B-M correlation is not confirmed for NGC 4258.

Polarization spectra can constrain magnetic field parameters.

The method helps refine models of accretion disk magnetism.

Abstract

We consider the integral light polarization from optically thick accretion disks. Basic mechanism is the multiple light scattering on free electrons (Milne's problem) in magnetized atmosphere. The Faraday rotation of the polarization plane changes both the value of integral polarization degree $p$ and the position angle $\chi $. Besides, the characteristic spectra of these values appear. We are testing the known relation between magnetic field of black hole at the horizon $B_{BH}$ and its mass $M_{BH}$, and the usual power-law distribution inside the accretion disk. The formulae for $p(\lambda)$ and $\chi(\lambda)$ depend on a number of parameters describing the particular dependence of magnetic field in accretion disk (the index of power-law distribution, the spin of the black hole, etc.). Comparison of our theoretical values of $p$ and $\chi $ with observed polarization can help us to choice more realistic values of parameters if the accretion disk mechanism gives the main contribution to the observed integral polarization. The main content is connected with estimation of validity of the relation between $B_{BH}$ and $M_{BH}$. We found for the AGN NGC 4258 that such procedure does not confirm the mentioned correlation between magnetic field and mass of black hole.