Polarization calibration techniques for new-generation VLBI

TL;DR

This paper introduces new calibration software for VLBI polarization observations that addresses limitations of existing algorithms, enabling more accurate analysis with modern high-bandwidth, high-sensitivity arrays.

Contribution

The paper presents novel software capable of simultaneous multi-calibrator fitting, nonlinear instrumental polarization modeling, and self-calibration for polarization leakage in VLBI.

Findings

Improved calibration accuracy for modern VLBI arrays.

Ability to handle high bandwidth and dynamic range data.

Enhanced modeling of instrumental polarization effects.

Abstract

The calibration and analysis of polarization observations in Very Long Baseline Interferometry (VLBI) requires the use of specific algorithms that suffer from several limitations, closely related to assumptions in the data properties that may not hold in observations taken with new-generation VLBI equipment. Nowadays, the instantaneous bandwidth achievable with VLBI backends can be as high as several GHz, covering several radio bands simultaneously. In addition, the sensitivity of VLBI observations with state-of-the-art equipment may reach dynamic ranges of tens of thousands, both in total intensity and in polarization. In this paper, we discuss the impact of the limitations of common VLBI polarimetry algorithms on narrow-field observations taken with modern VLBI arrays (from the VLBI Global Observing System, VGOS, to the Event Horizon Telescope, EHT) and present new software that overcomes these limitations. In particular, our software is able to perform a simultaneous fit of multiple calibrator sources, include non-linear terms in the model of the instrumental polarization and use a self-calibration approach for the estimate of the polarization leakage in the antenna receivers.