Murriyang cryogenic phased array feed: spectral-line results and noise-reduction methods

TL;DR

The paper presents the first spectral-line commissioning results for the cryogenic PAF (cryoPAF) on the Murriyang Parkes telescope, highlighting a 72-beam, 0.7–2 GHz system with high spectral resolution and strong on-dish sensitivity suitable for HI mapping and FRB work. It develops and tests high-dimensional, unsupervised denoising methods based on higher-order tensor SVD (e.g., CPSVD, TuckerSVD) to reduce noise and foreground contamination while preserving faint signals, validated through injections of compact and extended HI signals. Science-verification HI observations of NGC 6744 and the LMC show good agreement with previous data, and the LMC analysis reveals a potential low-column-density HI component not captured by earlier surveys, illustrating the instrument’s enhanced sensitivity. The results demonstrate that 3D tensor SVD techniques substantially improve signal recovery for both compact and extended HI sources, establishing a path toward robust HI intensity mapping with cryoPAF and informing future data-analysis strategies in the presence of RFI and strong continuum structure.

Abstract

Spectral-line results from a new cryogenic phased array feed (cryoPAF) on the Murriyang telescope at Parkes are presented. This array offers a significant improvement in field of view, aperture efficiency, bandwidth, chromaticity and survey speed compared with conventional horn-fed receivers. We demonstrate this with measurements of sky calibrators and observations of 21-cm neutral hydrogen (HI) in the LMC and the nearby galaxy NGC 6744. Within 0.3 deg of the optical axis, the ratio of system temperature to dish aperture efficiency is 25 K and the ratio with beam efficiency is 21 K (at 1.4 GHz). For the previously measured $T_{sys} = 17$ K, respective efficiency values 0.7 and 0.8 are derived. Our HI observational results are in good agreement with previous results, although detailed comparison with multibeam observations of the LMC suggests that the earlier observations may have missed an extended component of low-column-density gas ($8\times 10^{18}$ cm$^{-2}$). We use the cryoPAF zoom-band and wideband data to make a preliminary investigation of whether the large number of simultaneous beams (72) permits the use of novel data reduction methods to reduce the effects of foreground/background continuum contamination and RFI. We also investigate if these methods can better protect against signal loss for the detection of faint, extended cosmological signals such as HI intensity maps. Using robust higher-order singular value decomposition (SVD) techniques, we find encouraging results for the detection of both compact and extended sources, including challenging conditions with high RFI occupancy and significant sky continuum structure. Examples are shown that demonstrate that 3D SVD techniques offer a significant improvement in noise reduction and signal capture compared with more traditional layered 2D techniques.

Figures (18)

• Figure 1: The cryoPAF closepack72 beam footprint defining the arrangement of beams within the field of view. When the feed angle is set to 0 deg, the X and Y offset correspond to the sky offsets in azimuth and elevation, respectively, with respect to the optical axis of the telescope (beam 71 for this footprint). The radius of each blue circle is 0.1 deg; the hatched red circle represents the approximate beam size at 1.4 GHz ($\sim 0.24$ deg).
• Figure 2: The $7\times7$ pointing pattern used to observe the LMC. There was no parallactification of the cryoPAF during these observations, so the fields are rotated with respect to Figure \ref{['fig:closepack72']}. The red cross indicates the position of the archival multibeam spectrum discussed in the text.
• Figure 3: A comparison of HI spectra at a similar position within the LMC (RA = $05^{\rm h}07^{\rm m}10^{\rm s}$, Dec = $-69^{\circ}14'41"$, J2000), as marked with the red cross in Figure \ref{['fig:LMCpointings']}. The red spectrum is a 5-s archival P312 multibeam spectrum from 1998 December 16 2003MNRAS.339...87S. The blue spectrum is 10 s of data from cryoPAF beam 19 from 2024 November 24, smoothed to a similar resolution as the multibeam data (3.9 kHz). Both are Stokes $I$ spectra. No bandpass calibration or baseline fitting has been applied to either spectrum. The cryoPAF brightness temperature and barycentric frequency scales are approximately correct. The multibeam spectrum has an arbitrary temperature scale, with no barycentric correction applied.
• Figure 4: System temperature measurements taken on 2024 November 18 for all 72 cryoPAF beams and both orthogonal polarisations at 1.4 GHz from calibrations using (top) the flux density calibrator PKS B1934-638 and (bottom) the Galactic HI source S9. The B1934-638 measurement includes a dish efficiency term ($\eta_{\rm d}$) which decreases away from the optical axis. The S9 measurement includes a main beam efficiency term ($\eta_{\rm mb}$), which decreases less quickly away from the optical axis.
• Figure 5: A spatially integrated cryoPAF HI spectrum of NGC 6744, compared with an integrated spectrum from the HIPASS Bright Galaxy Catalogue 2004AJ....128...16K. The cryoPAF spectrum has been Hanning smoothed to a resolution of 75 kHz; the HIPASS resolution is 85 kHz.