Magnetic Fields of Agns and Standard Accretion Disk Model: Testing by Optical Polarimetry

TL;DR

This study introduces a method to estimate supermassive black hole magnetic fields using optical polarization data, testing models with observational data from NGC 4258 to determine field strengths and disk properties.

Contribution

The paper develops a novel approach to estimate SMBH magnetic fields via optical polarimetry, comparing regular and turbulent magnetic field models, and applies it to NGC 4258.

Findings

Estimated magnetic field strength at SMBH horizon: 10^3 - 10^4 Gauss.

The turbulent magnetic field model is more suitable for NGC 4258.

Wavelength-dependent polarization can refine accretion disk parameters.

Abstract

We have developed the method that allows us to estimate the magnetic field strength at the horizon of a supermassive black hole (SMBH) through the observed polarization of optical emission of the accreting disk surrounding SMBH. The known asymptotic formulae for the Stokes parameters of outgoing radiation are azimuthal averaged, which corresponds to an observation of the disk as a whole. We consider two models of the embedding 3D-magnetic field, the regular field, and the regular field with an additional chaotic (turbulent) component. It is shown that the second model is preferable for estimating the magnetic field in NGC 4258. For estimations we used the standard accretion disk model assuming that the same power-law dependence of the magnetic field follows from the range of the optical emission down to the horizon. The observed optical polarization from NGC 4258 allowed us to find the values 10^3 - 10^4 Gauss at the horizon, depending on the particular choice of the model parameters. We also discuss the wavelength dependencies of the light polarization, and possibly applying them for a more realistic choice of accretion disk parameters.