Polarization of radiation of point-like source reflected from turbulent magnetized atmosphere

Free access article

Issue		A&A Volume 481, Number 1, April I 2008


Page(s)		217 - 228
Section		Stellar atmospheres
DOI		http://dx.doi.org/10.1051/0004-6361:20078828
Published online		25 January 2008

A&A 481, 217-228 (2008)
DOI: 10.1051/0004-6361:20078828

Polarization of radiation of point-like source reflected from turbulent magnetized atmosphere

N. A. Silant'ev¹ and Yu. N. Gnedin²

¹ Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro 1, Apartado Postal 51 y 216, 72840, Tonantzintla, Pue., México
e-mail: silant@inaoep.mx
² Central Astronomical Observatory at Pulkovo of the Russian Academy of Sciences, 196140 St.-Petersburg, Pulkovo, Russia

(Received 10 October 2007 / Accepted 7 December 2007)

Abstract
We consider the multiple scattering of the light from a point-like source located above the semi-infinite electron, turbulent, and magnetized atmospheres. The frozen magnetic field has both the regular $\vec{B}_0$ and stochastic $\vec{B'}$ components ( $\vec{B}= \vec{B}_0+ \vec{B'}$ ). The stochastic Faraday rotations due to fluctuations $\vec{B'}$ decrease the intensity of each separate polarized beam (the extinction factor is proportional to $\lambda^4\langle B'^2\rangle$ ). This decrease at large $\lambda$ dominates the usual decrease ( $\propto$ $\lambda^2B_0\cos\Theta_0$ ) caused by summing beams with very different Faraday's rotation angles. This effect changes the spectrum of polarization degree as compared with what is influenced by the regular magnetic field. We calculated the integral (observed) polarization of the reflected radiation with the inclusion of unpolarized radiation going directly from the point-like source. We present the observed polarization for various degrees of true absorption of the radiation into the atmosphere and the values of magnetic energy fluctuations. The spectra of polarization in the optical ( $\lambda =0$ -1 $\mu\rm m$ ), infrared ( $\lambda =1$ -5 $\mu\rm m$ ), and X-ray (E=1-50 keV) regions of the wavelengths are presented. We discuss the possibility of estimating parameters of magnetic field fluctuations from the observation of the spectra of polarization in AGNs with the X-ray excesses and in the turbulent accretion disk in NGC 4258.

Key words: radiative transfer -- turbulence -- magnetic fields -- polarization

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