Constraining Theories of Polarized SiO Maser Transport: Multi-Epoch Analysis of a $\pi/2$ Electric Vector Rotation Feature

Abstract

The detailed polarization mechanisms of SiO masers originating from the near circum\-stellar environment of Asymptotic Giant Branch stars are not yet definitively known. Prevailing theories are broadly classified as either Zeeman or non-Zeeman in origin, the latter including effects such as anisotropic pumping or anisotropic resonant scattering. The predicted behavior of the linear and circular polarization fractions and electric vector position angle vary by theory. In particular, individual maser features that exhibit a rotation in linear polarization of $\sim \pi/2$ as a function of frequency over their extent can be utilized as a test of several maser polarization transport theories. In this paper, we analyze one SiO ($\nu=1$, $J=1-0$) maser feature toward the Mira variable, TX Cam that exhibits this internal polarization rotation and persists across five epochs (spanning $\sim3$ months). We compare our results to the predictions by several maser polarization theories and find that the linear polarization across the feature is consistent with a geometric effect for a saturated maser originating when the angle between the projected magnetic field and the line of sight ($\theta$) crosses the Van Vleck angle $\theta_F \sim 55^{\circ}$. However, the electric vector position angle (EVPA) exhibits a smooth rotation across the spatial extent of the feature rather than the expected abrupt $\pi/2$ flip. We discuss possible explanations for this discrepancy and alternative theories. Circular polarization across the feature is also analyzed and it is the most accurately described by Zeeman effects giving rise to a circular polarization fraction of the form $m_c$ is approximately proportional to $\cos \theta$.