Overview

The Max-Planck-Institut für Radioastronomie (MPIfR) and the Max-Planck-Society, in partnership with SARAO, have equipped MeerKAT with S-band receivers to complement the UHF and L-band receiver systems, providing coverage in the 1.75 - 3.5 GHz frequency range. These receivers are now available for science in shared-risk mode.

Basic characteristics of the system are known, and limited list of calibrators have been characterised. Investigation is still under way regarding the following:

• expanding the S-band calibrator list

• off-axis polarisation performance and calibration

On the whole, the system is capable of high sensitivity imaging in Stokes I (see commissioning reports below). We note that polarisation leakage is significant in the upper half of the band and at this stage polarimetric observations in the S3 and S4 sub-bands are not advised - see discussion below.

High dynamic range imaging will require both self-calibration and direction-dependent corrections due to pointing irregularities which are enhanced at higher frequencies.

Sub-bands

While the receiver and digitizer bandwidths are 1.75 GHz, the MeerKAT correlator can only process 875 MHz at a time. Digital band selection allows the band to be centred at 2.2, 2.4, 2.6, 2.8 or 3.1 GHz.

Sensitivity

The mean SEFDs per antenna are given in Figures 1 and 2 below. Note that these are preliminary results, yet to be refined by gain curves.

Open

Figure 1: Mean SEFD per antenna across the full receiver band.

Open

Figure 2: Mean SEFD per antenna for each sub-band.

Commissioning results

Flux scale

The Reynolds (1994) and Partridge et al. (2016) flux scales were compared to the Perley-Butler (2017) flux scales in this report. The derived flux errors are generally less than 2% but we note large gain variations between observations in the S3 and S4 sub-bands, likely due to pointing errors. Work is currently under way to improve mechanical pointing and we expect that the situation has improved since these data were obtained.

Noise-limited imaging

An observation of the ‘DEEP2’ field was done in May 2021 in the S4 sub-band using 42 antennas. The on-source time was 7.5 hours. No significant issues were noted and the rms noise appears to be as expected (see Fig. 3).

Open

Figure 3: Total intensity image of the DEEP2 field observed with MeerKAT in the S4 subband. The 1^◦ × 1^◦ image has a PSF whose size is 3“ × 2”.8 and the rms noise is about 3 µJy beam^-1. No primary beam correction has been applied to this image. The unit on the colorbar is µJy beam^-1.

Astrometry

A test observation of the COSMOS field at S4 compared to the VLA 3 GHz COSMOS survey shows astrometric offsets are ∼2% of a synthesised beam width at worst (I. Heywood priv. comm).

High dynamic range imaging

Two observations were conducted on the field containing the “Corkscrew” galaxy, an extremely bright object with complex structure (see Fig. 4). Strong artefacts were initially found near the bright core in the first observation.

Better results were found when centering the pointing position on the bright core. This reduces gain variations that would be introduced by pointing offsets. Direction dependent calibration produced significant improvement in image quality and dynamic range. More details can be found in this report.

Open

Figure 4: Negative grayscale image of central portion of a field observed with MeerKAT at in the S4 sub-band showing the several extended sources in the field. The “Corkscrew” is on the top left and the pointing center was on the compact source on the western side of the galaxy. The image has been amplitude and phase self calibrated and the bright sources in the upper left and lower right peeled to reduce the effects of DDEs. The stretch is -10 to 500 µJy/beam and the field shown is 37′ × 32′ .

Spectral line capabilities

Note that only the ‘wideband’ 32K channel mode is available at this time.

Test observations were made of three CH hyperfine-structure (hfs) transitions of CH at 3.3 GHz (in the S4 sub-band) towards Sgr B2, G09.622+0.19 and W51. Good agreement was found between the MeerKAT and previous VLA observations in both flux density and measured velocities.

Observations of transients

The ThunderKAT project conducted an observation of an X-ray binary and concluded that “Overall, the imaging capabilities at this high frequency with MeerKAT are impressive and although the targeted field is complex, the low RFI levels at this part of the band help to reach high sensitivity at ∼3 arcsec angular resolution.”

RFI

The dominant source of RFI appears to be from the Globalstar satellite constellation at 2483.5 - 2495.0 MHz (in the S3 sub-band), but this affects mainly the short baselines. Baseline dependent flagging should be used.

We are in the process of building up statistics on other less frequent transmissions, and will update the RFI page when we have more information.

Polarimetry

While plausible results have been obtained on a test source (see section V of the report on the Corkscrew Galaxy), we note that there is potentially high leakage in the S3 and S4 bands.

Users interested in doing sensitive polarimetric measurements, particularly across an extended area, are advised to avoid the higher frequency sub-bands. See De Villiers (2023) for more technical details.

Acknowledgments