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A new 1.4 GHz 19-element, dual-polarization, cryogenic phased array feed(PAF) radio astronomy receiver has been developed for the Robert C. Byrd GreenBank Telescope (GBT) as part of FLAG (Focal L-band Array for the GBT) project.Commissioning observations of calibrator radio sources show that this receiverhas the lowest reported beamformed system temperature ($T_{\rm sys}$)normalized by aperture efficiency ($\eta$) of any phased array receiver todate. The measured $T_{\rm sys}/\eta$ is $25.4 \pm 2.5$ K near 1350 MHz for theboresight beam, which is comparable to the performance of the current 1.4 GHzcryogenic single feed receiver on the GBT. The degradation in $T_{\rmsys}/\eta$ at $\sim$ 4 arcmin (required for Nyquist sampling) and $\sim$ 8arcmin offsets from the boresight is, respectively, $\sim$ 1\% and $\sim$ 20\%of the boresight value. The survey speed of the PAF with seven formed beams islarger by a factor between 2.1 and 7 compared to a single beam system dependingon the observing application. The measured performance, both in frequency andoffset from boresight, qualitatively agree with predictions from a rigorouselectromagnetic model of the PAF. The astronomical utility of the receiver isdemonstrated by observations of the pulsar B0329+54 and an extended HII region,the Rosette Nebula. The enhanced survey speed with the new PAF receiver willenable the GBT to carry out exciting new science, such as more efficientobservations of diffuse, extended neutral hydrogen emission from galacticin-flows and searches for Fast Radio Bursts.

Performance of a highly sensitive, 19-element, dual-polarization, cryogenic L-band Phased Array Feed on the Green Bank Telescope