Commissioning the HI Observing Mode of the Beamformer for the Cryogenically Cooled Focal L-band Array for the GBT (FLAG)

TL;DR

This paper reports on the commissioning of a new digital beamforming back end for the FLAG array on the GBT, demonstrating its high sensitivity, stability, and suitability for spectral line observations.

Contribution

It introduces a custom software and methodology for beamforming with the FLAG array, establishing it as a leading PAF receiver for radio astronomy.

Findings

FLAG achieves the lowest T_sys/eta among PAFs to date.

Beam weights remain stable over time.

Sensitivity and noise properties are comparable or superior to existing receivers.

Abstract

We present the results of commissioning observations for a new digital beamforming back end for the Focal plane L-band Array for the Robert C. Byrd Green Bank Telescope (FLAG), a cryogenically cooled Phased Array Feed (PAF) with the lowest measured T_sys/eta of any PAF outfitted on a radio telescope to date. We describe the custom software used to apply beamforming weights to the raw element covariances to create research quality spectral line images for the new fine-channel mode, study the stability of the beam weights over time, characterize FLAG's sensitivity over a frequency range of 150 MHz, and compare the measured noise properties and observed distribution of neutral hydrogen emission from several extragalactic and Galactic sources with data obtained with the current single-pixel L-band receiver. These commissioning runs establish FLAG as the preeminent PAF receiver currently available for spectral line observations on the world's major radio telescopes.