K-band AstroGeo VLBI Programs
The K-band Astronomy VLBI Program
Both astrometric and geodetic Very Long Baseline Interferometry (VLBI) rely on regular and accurate measurements of the positions of Active Galactic Nuclei (AGN) to maintain and improve the International Celestial and Terrestrial Reference Frames (ICRF and ITRF) and generate Earth orientation parameters (EOP). Simultaneously, many astronomical VLBI studies of AGN necessitate frequent and dense monitoring of the same objects. By combining astrometric, geodetic and astronomical VLBI observations we have a remarkable opportunity to optimise telescope resources, save time, and drive scientific progress in all these fields. To this end, we initiated a pilot project in September 2023 to test the feasibility of such a joint large-scale observing program. This program entails weekly K-band (24 GHz) observations involving the Korean VLBI Network (KVN) antennas, the Hartebeesthoek (HartRAO) 26-m antenna in South Africa, the Mopra 22-m antenna in Australia, the Sejong 20-m geodetic antenna in Korea, the Hobart 26-m antenna in Australia, and the Yebes 40-m in Spain.
Under our K-band Astronomy VLBI Program we aim to make our database of multi-epoch K-band images and light-curves (e.g. de Witt et al., 2023), obtained from our astrometric observing sessions, available to the astronomical community. We also aim to promote collaboration between astronomy, astrometry, and geodesy, particularly combining astronomical, astrometric and geodetic observing programs, where possible, for global VLBI monitoring of AGN. Our overarching goal is to accommodate diverse scientific needs and to generate comprehensive and widely accessible data products for the scientific community’s benefit.”
The K-band AstroGeo VLBI program will provide a wealth of data that can be used for many different scientific applications. Detailed below are some programs that will use the data from an astronomical perspective:
Cosmological QUOKKAS:
This is a project that leverages the unique properties of quasars (specifically, their rapid radio variability), to measure cosmic distances. At the heart of the project is the innovative "Causality Distance" method, a single-rung approach that uses the speed of light as a cosmic standard ruler (Hodgson et al., 2020). This method allows for the estimation of distances across a wide range of redshifts, from nearby galaxies to distant quasars. By observing the apparent size as measured using VLBI and then using radio variability to estimate the 'true' size of an emitting region in a quasar. The ratio of these quantities gives the distance. The method was applied concretely on the radio galaxy 3C 84, the bright radio source at the centre of the Perseus Cluster. Using data from the Boston University blazar monitoring program, the team was able to independently measure the Hubble Constant, offering valuable insights into the expansion rate of the Universe.
Extensive analysis of archival data within the QUOKKAS project revealed the importance of resolved sources, prominent flares, and well-sampled light curves, highlighting the need for regular global VLBI monitoring of AGN. Coincidentally, all potential sources earmarked for monitoring within the QUOKKAS project are also part of the K-band CRF. Consequently, a subset of 35 bright AGNs spanning various redshifts has been meticulously selected and subjected to weekly monitoring during routine K-CRF astrometric VLBI sessions. Consequently, the high-cadence observations within the K-CRF initiative play a pivotal role in ensuring the success of the QUOKKAS project.
Further advancements in the project addressed the challenge of relativistic effects in VLBI sources. By positing a minimum intrinsic brightness temperature for these sources, the team developed a method to correct for these relativistic effects, potentially enhancing the precision of distance measurements. The promise of this method is such that with continued observation, it could rival other distance measures (such as Type Ia supernovae) within the next decade. The Cosmological QUOKKAS project is set to make a significant impact on our understanding of the cosmos, refining our measurement techniques, and expanding our knowledge of the universe's expansion.
Multi-wavelength cross-correlations in AGN:
This is a project that revolves around exploring multi-wavelength cross-correlations in AGN, in particular between the flux density times series (light curves) constructed from the K-band CRF data and the gamma-ray photon flux light curves from the Fermi Large Area Telescope (Fermi-LAT). The goal is to evaluate the temporal delay (time lags) between radio and gamma-ray flaring events which in turn may shed light on the physical mechanisms driving the generation of non-thermal radiation as well as the dominant production mechanism and the exact location of the gamma-ray emission — a facet of AGN studies that is still poorly understood.
The strength of the K-band CRF observations lies in its high cadence and continuous monitoring of radio flux densities (see e.g. the time-series of the source NRAO140 at the bottom of this page) across a substantial AGN sample. This is complemented by VLBI imaging performed concurrently with flux density monitoring. The VLBI images are invaluable for constructing integrated light curves for the entire source and providing critical spatial information, such as the location of the emission and the amount of structure on mas/sub-mas scales. This project will also examine the relationship between source position stability, obtained from the K-CRF astrometric analysis, and the astrophysical properties of these AGNs.
Time-series plots of the source NRAO140 (J0336+3218) across 73 distinct epochs of VLBA observations between July 2015 and January 2023. The top panel shows the CLEAN and core flux density, and the peak brightness. The bottom panel shows the weighted average correlated flux density for four baseline length ranges.