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

Abstract

This paper considers the problem of accurate measurement of circular polarization in imaging spectral-line VLBI observations in the λ = 7 mm and λ = 3 mm wavelength bands. This capability is especially valuable for the full observational study of compact, polarized SiOmaser 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 Ë = 7 mm and derive an estimate of the predicted accuracy of the method of mc ≤ 0.5% or better at λ= 7 mmand mc ≥ 0.5.1% or better at λ 3 mm. Both the strengths and weaknesses of the proposed algorithm are discussed, along with suggestions for future work. © 2011 ESO.