Issue A&A Volume 507, Number 1, November III 2009 Page(s) 171 - 182 Section Extragalactic astronomy DOI 10.1051/0004-6361/200810892 Published online 08 September 2009

A&A 507, 171-182 (2009)
DOI: 10.1051/0004-6361/200810892

Magnetic fields of AGNs and standard accretion disk model: testing by optical polarimetry

N. A. Silant'ev¹, M. Yu. Piotrovich², Yu. N. Gnedin², and T. M. Natsvlishvili<sup>2

1</sup>  Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro 1, Apartado Postal 51 y 216, 72840, Tonantzintla, Puebla, México
    e-mail: silant@inaoep.mx
²  Central Astronomical Observatory at Pulkovo of Russian Academy of Sciences, 196140, Saint-Petersburg, Pulkovskoe shosse 65, Russia

Received 1 September 2008 / Accepted 15 July 2009

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 10³-10⁴ 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.


Key words: black hole physics -- magnetic fields -- accretion, accretion disks -- polarization

© ESO 2009

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