Calibrating VLBI Polarization Data Using GPCAL. II. Time-Dependent Calibration

TL;DR

This paper introduces a novel time-dependent calibration method for VLBI polarization data, implemented in GPCAL, which effectively corrects off-axis leakages and improves the quality of linear polarization imaging.

Contribution

The paper presents a new time-dependent instrumental polarization calibration technique integrated into GPCAL, addressing off-axis leakages that vary over time in VLBI data.

Findings

Successfully reproduces large amplitude off-axis leakages in synthetic data.

Shows consistent off-axis leakage trends over time in real VLBA data.

Significantly reduces rms-noise in polarization images, enhancing image quality.

Abstract

We present a new method of time-dependent instrumental polarization calibration for Very Long Baseline Interferometry (VLBI). This method has been implemented in the recently developed polarization calibration pipeline GPCAL. Instrumental polarization, also known as polarimetric leakage, is a direction-dependent effect, and it is not constant across the beam of a telescope. Antenna pointing model accuracy is usually dependent on time, resulting in off-axis polarimetric leakages that can vary with time. The method is designed to correct for the off-axis leakages with large amplitudes that can severely degrade linear polarization images. Using synthetic data generated based on real Very Long Baseline Array (VLBA) data observed at 43 GHz, we evaluate the performance of the method. The method was able to reproduce the off-axis leakages assumed in the synthetic data, particularly those with large amplitudes. The method has been applied to two sets of real VLBA data and the derived off-axis leakages show very similar trends over time for pairs of nearby sources. Furthermore, the amplitudes of the off-axis leakages are strongly correlated with the antenna gain correction factors. The results demonstrate that the method is capable of correcting for the off-axis leakages present in VLBI data. By calibrating time-dependent instrumental polarization, the rms-noise levels of the updated linear polarization images have been significantly reduced. The method is expected to substantially enhance the quality of linear polarization images obtained from existing and future VLBI observations.