HI 21cm Cosmology and the Bi-spectrum: Closure Diagnostics in Massively Redundant Interferometric Arrays

TL;DR

This paper explores the use of closure phase spectra in massively redundant interferometric arrays, specifically the Hydrogen Epoch of Reionization Array, to diagnose non-redundancies and identify problematic antennas, aiding in the search for cosmic reionization signals.

Contribution

It introduces a novel method utilizing closure phase spectra to assess array redundancy and detect issues without relying on calibration, demonstrated on real data from the HERA array.

Findings

Median absolute deviation from redundancy in closure phase is about 4.5 degrees.

Closure phase spectra can identify ill-behaved antennas independently of calibration.

Closure spectra are stable over time at the same local sidereal time, aiding in array diagnostics.

Abstract

New massively redundant low frequency arrays allow for a novel investigation of closure relations in interferometry. We employ commissioning data from the Hydrogen Epoch of Reionization Array to investigate closure quantities in this densely packed grid array of 14m antennas operating at 100 MHz to 200 MHz. We investigate techniques that utilize closure phase spectra for redundant triads to estimate departures from redundancy for redundant baseline visibilities. We find a median absolute deviation from redundancy in closure phase across the observed frequency range of about 4.5deg. This value translates into a non-redundancy per visibility phase of about 2.6deg, using prototype electronics. The median absolute deviations from redundancy decrease with longer baselines. We show that closure phase spectra can be used to identify ill-behaved antennas in the array, independent of calibration. We investigate the temporal behavior of closure spectra. The Allan variance increases after a one minute stride time, due to passage of the sky through the primary beam of the transit telescope. However, the closure spectra repeat to well within the noise per measurement at corresponding local sidereal times (LST) from day to day. In future papers in this series we will develop the technique of using closure phase spectra in the search for the HI 21cm signal from cosmic reionization.