Circular polarization measurement in millimeter-wavelength spectral-line VLBI observations

TL;DR

This paper develops an advanced calibration algorithm for accurate circular polarization measurement in millimeter-wavelength spectral-line VLBI observations, crucial for studying magnetic fields and maser physics around evolved stars.

Contribution

It introduces novel enhancements to existing VLBI polarimetry methods, improving accuracy in low SNR conditions at 3mm and 7mm wavelengths.

Findings

Achieves polarization measurement accuracy of <0.5% at 7mm

Achieves polarization measurement accuracy of <0.5-1% at 3mm

Provides a detailed analysis of instrumental factors affecting measurements

Abstract

This paper considers the problem of accurate measurement of circular polarization in imaging spectral-line VLBI observations in the lambda=7 mm and lambda=3 mm wavelength bands. This capability is especially valuable for the full observational study of compact, polarized SiO maser components in the near-circumstellar environment of late-type, evolved stars. Circular VLBI polarimetry provides important constraints on SiO maser astrophysics, including the theory of polarized maser emission transport, and on the strength and distribution of the stellar magnetic field and its dynamical role in this critical circumstellar region. We perform an analysis here of the data model containing the instrumental factors that limit the accuracy of circular polarization measurements in such observations, and present a corresponding data reduction algorithm for their correction. The algorithm is an enhancement of existing spectral line VLBI polarimetry methods using autocorrelation data for calibration, but with innovations in bandpass determination, autocorrelation polarization self-calibration, and general optimizations for the case of low SNR, as applicable at these wavelengths. We present an example data reduction at $\lambda=7$ mm and derive an estimate of the predicted accuracy of the method of m_c < 0.5% or better at lambda=7 mm and m_c < 0.5-1% or better at lambda=3 mm. Both the strengths and weaknesses of the proposed algorithm are discussed, along with suggestions for future work.