Characterizing maser polarization: effects of saturation, anisotropic pumping, and hyperfine structure

Abstract

Context. The polarization of masers contains information on the magnetic field strength and direction of the regions they occur in. Many maser polarization observations have been performed over the last 30 years. However, versatile maser polarization models that can aide in the interpretation of these observations are not available. Aims. We developed a program suite that can compute the polarization by a magnetic field of any non-paramagnetic maser species at arbitrarily high maser saturation. Furthermore, we investigated the polarization of masers by non-Zeeman polarizing effects. We present a general interpretive structure for maser polarization observations. Methods. We expanded existing maser polarization theories of non-paramagnetic molecules and incorporated them in a numerical modeling program suite. Results. We present a modeling program called CHAracterizes Maser Polarization (CHAMP) that can examine the polarization of masers of arbitrarily high maser saturation and high angular momentum. Hyperfine multiplicity of the maser-transition can also be incorporated. The user is able to investigate non-Zeeman polarizing mechanisms such as anisotropic pumping and polarized incident seed radiation. We present an analysis of the polarization of v = 1 SiO masers and the 22 GHz water maser. We comment on the underlying polarization mechanisms, and also investigate non-Zeeman effects. Conclusions. We identify the regimes where different polarizing mechanisms will be dominant and present the polarization characteristics of the SiO and water masers. From the results of our calculations, we identify markers to recognize alternative polarization mechanisms. We show that comparing randomly generated linear versus circular polarization (pL − pV) scatter-plots at fixed magnetic field strength to the observationally obtained pL − pV scatter can be a promising method of ascertaining the average magnetic field strength of a large number of masers.

Polarisation properties of methanol masers

“…To investigate the polarisation of methanol masers by a magnetic field and its effects on the hyperfine structure, we ran simulations using the CHAMP code (Lankhaar & Vlemmings 2019). CHAMP is a maser polarisation radiative transfer code that takes into account all dominant hyperfine components of a molecule and their individual Landé factors.…”

“…We ran calculations assuming an intrinsic thermal velocity width v th = 1 km s −1 . For all the transitions, we used a pumping efficiency of δ = 0.02, a decay rate of the upper and lower level of Γ = 1 s −1 (Vlemmings et al 2010;Lankhaar & Vlemmings 2019), and in the case of anisotropic pumping, the anisotropy degree, = 0.01. The parameters are described in Lankhaar & Vlemmings 2019.…”

“…Thanks to recent detailed calculations of the Landé g-factors for all methanol transitions and the associated hyperfine components (Lankhaar et al 2016(Lankhaar et al , 2018, it is now possible to obtain a complete interpretation of the methanol maser polarisation properties and infer the magnetic field characteristics. A&A 644, A122 (2020) In this paper, we investigate the methanol maser polarisation properties using the maser polarisation radiative transfer code, CHAMP (Lankhaar & Vlemmings 2019), performing several simulations of different methanol masers transitions, as described in Sect. 3.…”

Characterizing maser polarization: effects of saturation, anisotropic pumping, and hyperfine structure (Corrigendum)

“…This error has no influence on the total maser brightness, the total polarization fraction, or the circular polarization, but it leads to an erroneous polarization angle. The polarization angle that we report in Lankhaar & Vlemmings (2019), χ, is related to the correct polarization angle, χ , by…”

“…The polarization angles that are reported in Lankhaar & Vlemmings (2019) have to be revisited. It has been pointed out to us that a bug in our code lead to the wrong sign of the Stokes parameters…”