Polarization calibration techniques for the new-generation VLBI

¹ Departament d’Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, 46100 Burjassot, València, Spain
e-mail: i.marti-vidal@uv.es
² Observatori Astronòmic, Universitat de València, Parc Científic, C. Catedràtico José Beltrán 2, 46980 Paterna, València, Spain
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
⁵ Centro Astronómico de Yebes (IGN), Apartado 148, 19180 Yebes, Spain

Received: 25 September 2020
Accepted: 7 December 2020

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. Currently, the instantaneous bandwidth achievable with VLBI backends can be as high as several gigahertz, covering several radio bands simultaneously. In addition, the sensitivity of VLBI observations with the most updated equipment may reach dynamic ranges of tens of thousands, both in total intensity and 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 to the Event Horizon Telescope, and present new software that overcomes these limitations. In particular, our software is able to perform a simultaneous fit of multiple calibrator sources, include nonlinear 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.