GPCAL: a generalized calibration pipeline for instrumental polarization in VLBI data

TL;DR

GPCAL is an automated, generalized pipeline for instrumental polarization calibration in VLBI data, improving accuracy by fitting complex models and utilizing detailed calibrator polarization data across multiple frequencies.

Contribution

It introduces a novel calibration pipeline that models second-order instrumental polarization terms and uses advanced calibrator polarization models, surpassing traditional assumptions.

Findings

Successfully calibrates VLBI data across multiple arrays and frequencies.

Reproduces complex polarization structures consistent with previous studies.

Reveals detailed polarization in AGN jets, including 3C 273.

Abstract

We present the Generalized Polarization CALibration pipeline (GPCAL), an automated pipeline for instrumental polarization calibration of very long baseline interferometry (VLBI) data. The pipeline is designed to achieve a high calibration accuracy by means of fitting the instrumental polarization model, including the second-order terms, to multiple calibrators data simultaneously. It also allows using more accurate linear polarization models of calibrators for D-term estimation compared to the conventional way that assumes similar linear polarization and total intensity structures. This assumption has widely been used in the existing packages for instrumental polarization calibration but could be a source of significant uncertainties when there is no suitable calibrator satisfying the assumption. We demonstrate the capabilities of GPCAL by using simulated data, archival Very Long Baseline Array (VLBA) data of many active galactic nuclei (AGN) jets at 15 and 43 GHz, and our Korean VLBI Network (KVN) observations of many AGN jets at 86, 95, 130, and 142 GHz. The pipeline could reproduce the complex linear polarization structures of several sources shown in the previous studies using the same VLBA data. GPCAL also reveals a complex linear polarization structure in the flat-spectrum radio quasar 3C 273 from the KVN data at all four frequencies. These results demonstrate that GPCAL can achieve a high calibration accuracy for various VLBI arrays.