Polarized Redundant-Baseline Calibration for 21 cm Cosmology Without Adding Spectral Structure

TL;DR

This paper reviews and improves redundant-baseline calibration methods for 21 cm cosmology, focusing on eliminating artificial spectral structures and extending the approach to dual-polarization instruments.

Contribution

It identifies and addresses calibration degeneracies that cause spectral artifacts, and generalizes the method to polarized measurements, enhancing calibration accuracy.

Findings

Identified sources of spectral structure from calibration degeneracies

Demonstrated methods to eliminate artificial spectral features

Extended calibration techniques to dual-polarization instruments

Abstract

21 cm cosmology is a promising new probe of the evolution of visible matter in our universe, especially during the poorly-constrained Cosmic Dawn and Epoch of Reionization. However, in order to separate the 21 cm signal from bright astrophysical foregrounds, we need an exquisite understanding of our telescopes so as to avoid adding spectral structure to spectrally-smooth foregrounds. One powerful calibration method relies on repeated simultaneous measurements of the same interferometric baseline to solve for the sky signal and for instrumental parameters simultaneously. However, certain degrees of freedom are not constrained by asserting internal consistency between redundant measurements. In this paper, we review the origin of these "degeneracies" of redundant-baseline calibration and demonstrate how they can source unwanted spectral structure in our measurement and show how to eliminate that additional, artificial structure. We also generalize redundant calibration to dual-polarization instruments, derive the degeneracy structure, and explore the unique challenges to calibration and preserving spectral smoothness presented by a polarized measurement.