K-band AstroGeo VLBI - High-Frequency Q/W-band Program

K-band AstroGeo VLBI Programs

The High-Frequency Q/W-band VLBI Program

Observations to explore the construction of celestial reference frames (CRFs) at even higher radio frequencies, such as Q (43 GHz) and W (86 GHz) bands, are being investigated. Unlike at K-band, the ionosphere at Q-band and W-band is negligible. The sources are also expected to appear more compact at these higher radio frequencies. However, we also expect the sources to be weaker at Q, and W-band and observations may be limited by efficiency and pointing issues.

A recent exploratory imaging-astrometry project to compare the structure of 453 ICRF sources at S, X, K, and Q bands (e.g. de Witt et al., 2022; Hunt et al., 2023), from near-simultaneous observations on the Very Long Baseline Array (VLBA), showed promising results for the construction of a CRF at Q-band. In addition, results from the Korean multi-frequency AGN survey with the KVN (MASK), performed as a Korean VLBI Network (KVN) legacy program to densify the existing VLBI catalogue of extragalactic radio sources at 22/43/86/129 GHz, have detected more than 600 sources at W-band, thus showing promising results for the construction of a CRF also at W-band.

If the construction of a CRF at Q- and W-bands is viable, the Korean K/Q/W-band (22/43/86-GHz) receiver system, now being installed at many new sites other than Korea, will allow for simultaneous construction of a K/Q/W CRF (e.g. Jung 2023). In addition, the technique of frequency phase transfer, enabled by the Korean multi-band receivers with shared optical path, will lead to an increase in the coherence time and a decrease in the phase noise at W-band, which in turn will allow for improved sensitivity and astrometric accuracy at W-band (Dodson et al., 2022).

Under our High-Frequency (Q/W-band) VLBI Program we aim to show whether the majority of our sources are bright enough to be detected at Q-band and whether they are more compact at Q-band vs. K-band. That is to say, will the factor of two increase in interferometer resolution benefit a Q-band CRF or will it expose a multi-component core that confuses the definition of the reference point? In short, what is the optimal observing band for a radio CRF? We hope to use the results from our campaign to determine if the pursuit of a Q-band CRF, and perhaps even a W-band CRF, is worthwhile.

Results from the VLBA imaging-astrometry project to compare the structure of ICRF sources at S, X, K, and Q bands are shown below:

The figure shows the S, X, K, and Q-band images of J0336+3218, from near-simultaneous VLBI observations taken with the VLBA in 2021. The maps are all on an absolute scale and the FWHM beam in lower left corner shows the resolution. Between S- and Q-band the resolution increase by almost a factor of 20. Image Credit: de Witt et al., 2022.

The figure shows the same S, X, K, and Q-band images of J0336+3218, from the figure above, but with the maps scaled so that the FWHM beam is the same size (i.e. the scale of each image is different). The weak extended structure at K-band is not a problem for CRF work, but the near-double core at X-band is. It is clear that the source becomes more compact, with the extended emission fading away, as one goes from S to Q-band. Image Credit: de Witt et al., 2022.