The radiative transfer of synchrotron radiation through a compressed random magnetic field

TL;DR

This paper derives simplified radiative transfer equations for synchrotron radiation in compressed, disordered magnetic fields, enabling easier interpretation of polarization in radio galaxy and AGN jet observations.

Contribution

It provides a simplified analytical approach to synchrotron radiative transfer in compressed magnetic fields, facilitating interpretation of polarization without full numerical solutions.

Findings

Polarization can increase even as magnetic field becomes less confined.

Transfer equations separate under certain conditions, simplifying calculations.

Results are relevant for understanding polarization in radio galaxy lobes and AGN jets.

Abstract

This paper examines the radiative transfer of synchrotron radiation in the presence of a magnetic field configuration resulting from the compression of a highly disordered magnetic field. It is shown that, provided Faraday rotation and circular polarization can be neglected, the radiative transfer equations for synchrotron radiation separate for this configuration, and the intensities and polarization values for sources that are uniform on large scales can be found straightforwardly in the case where opacity is significant. Although the emission and absorption coefficients must, in general, be obtained numerically, the process is much simpler than a full numerical solution to the transfer equations. Some illustrative results are given and an interesting effect, whereby the polarization increases while the magnetic field distribution becomes less strongly confined to the plane of compression, is discussed. The results are of importance for the interpretation of polarization near the edges of lobes in radio galaxies and of bright features in the parsec--scale jets of AGN, where such magnetic field configurations are believed to exist. (Note: The original ApJ version of this paper contained two errata, which are corrected in this version of the paper. First, Fig. 2 was plotted as a mirror image of the correct version, reflected about the line $\log_{10}(\nu/ \nu_0)=0$. Second, as stated, Equation A16 is valid for $\gamma=2$, not $\gamma=3$.)