Full Stokes polarimetric observations with a single-dish radio-telescope

TL;DR

This paper introduces a new calibration method for single-dish radio telescopes that enables accurate measurement of all four Stokes parameters, including circular polarization, by analyzing Mueller matrix elements.

Contribution

The authors develop a detailed calibration technique that improves the accuracy of Stokes V measurements in single-dish polarimetry, facilitating full polarization studies of astronomical sources.

Findings

Calibration method successfully applied to Effelsberg data

Stable D-terms confirm calibration reliability

Circular polarization measurements validated against known values

Abstract

The study of the linear and circular polarization in AGN allows one to gain detailed information about the properties of the magnetic fields in these objects. However, especially the observation of circular polarization (CP) with single-dish radio-telescopes is usually difficult because of the weak signals to be expected. Normally CP is derived as the (small) difference of two large numbers (LHC and RHC); hence an accurate calibration is absolutely necessary. Our aim is to improve the calibration accuracy to include the Stokes parameter V in the common single-dish polarimetric measurements, allowing a full Stokes study of the source under examination. A detailed study, up to the 2nd order, of the Mueller matrix elements in terms of cross-talk components allows us to reach the accuracy necessary to study circular polarization. The new calibration method has been applied to data taken at the 100-m Effelsberg radio-telescope during regular test observations of extragalactic sources at 2.8, 3.6, 6 and 11 cm. The D-terms in phase and amplitude appear very stable with time and the few known values of circular polarization have been confirmed. It is shown that, whenever a classical receiver and a multiplying polarimeter are available, the proposed calibration scheme allows one to include Stokes V in standard single-dish polarimetric observations as difference of two native circular outputs.