Polarized Maser Emission with In-Source Faraday Rotation

TL;DR

This paper develops a semi-classical model and numerical code to study polarization in astrophysical masers, focusing on the effects of small Zeeman splitting and Faraday rotation, and compares results with previous models.

Contribution

Introduces PRISM, a new numerical code for modeling polarized maser emission with in-source Faraday rotation, bridging different Zeeman splitting regimes.

Findings

Demonstrates smooth transition between small and wide Zeeman splitting cases.

Shows how Faraday rotation affects polarization angles and Stokes parameters.

Finds consistency with previous theoretical and computational models.

Abstract

We discuss studies of polarization in astrophysical masers with particular emphasis on the case where the Zeeman splitting is small compared to the Doppler profile, resulting in a blend of the transitions between magnetic substates. A semi-classical theory of the molecular response is derived, and coupled to radiative transfer solutions for 1 and 2-beam linear masers, resulting in a set of non-linear, algebraic equations for elements of the molecular density matrix. The new code, PRISM, implements numerical methods to compute these solutions. Using PRISM, we demonstrate a smooth transfer between this case and that of wider splitting. For a J=1-0 system, with parameters based on the $v=1, J=1-0$ transition of SiO, we investigate the behaviour of linear and circular polarization as a function of the angle between the propagation axis and the magnetic field, and with the optical depth, or saturation state, of the model. We demonstrate how solutions are modified by the presence of Faraday rotation, generated by various abundances of free electrons, and that strong Faraday rotation leads to additional angles where Stokes-Q changes sign. We compare our results to a number of previous models, from the analytical limits derived by Goldreich, Keeley and Kwan in 1973, through computational results by W. Watson and co-authors, to the recent work by Lankhaar and Vlemmings in 2019. We find that our results are generally consistent with those of other authors given the differences of approach and the approximations made.