Narrowband spectral line modes

Overview

A ‘zoom’ mode is available for higher resolution spectral line observations. It is designated as NE_107M for ‘narrowband extended’, and has a bandwidth of 107 MHz with 32k channels (with ~13.5 MHz rolloff on either side, giving an effective bandwidth of ~80 MHz – see Figure 1). This gives a channel width of 3.3 kHz or 0.7 km/s at 1.4 GHz. It runs concurrently with the wideband 4k channel correlator mode, providing simultaneous wideband continuum observations. It is being successfully used for the L-band LSPs MHONGOOSE and Fornax.

Figure 1: Bandpass response of the 107 MHz narrowband mode. The vertical lines indicate 4096 channels inwards from the edge. Users are advised to download and reduce channels 4096–28672 only. The ‘spike’ at 1420 MHz is due to Galactic HI.

 

The NE_54M mode (twice the frequency resolution of NE_107M) is available in shared risk mode at L band. Users interested in this capability should contact SARAO to discuss possible use cases.

Unfortunately the filter response causes 50% of the NE_54M mode band to be lost, giving an effective bandwidth of 27 MHz.

Figure 2: Bandpass response of the 54 MHz narrowband mode. The vertical lines indicate 8192 channels inwards from the edge. Users are advised to download channels 8192–24576 to reduce file sizes.

Commissioning results for the 107 MHz narrowband mode

In Figure 3 below we show the results of observations of M83 in both wideband and 107 MHz narrowband mode. The correlator is performing as expected and the expected sensitivity is achieved.

Figure 3: Images of M83 from the Local Volume HI Survey (LVHIS, Koribalski 2018) (top), the MeerKAT 32k wideband observation on the 8th of December 2019 (middle) and the MeerKAT 107 MHz narrowband observation on the 10th of April 2020 (bottom) with the same colour scales. The graphs left to right are the integrated value of the spectrum, the intensity weighted coordinate, and the intensity weighted dispersion of the coordinate. The contour levels in the middle plot are: 649, 617.5, 586, 554.6, 523.1, 491.7, 460.2, 428.7, 397.3, 365.8 km/s.

For more details see the commissioning reports from the Large Survey Projects Fornax and MHONGOOSE, which have been conducting extensive observations using this mode since August 2020:

MHONGOOSE commissioning: narrowband observations of NGC 5068

MeerKAT 32k zoom mode commissioning: NGC 1365