VLBI observations with the SKA-Mid at 12/15 GHz
Abstract
The SKA-Mid plans to support VLBI observations at frequencies up to 15 GHz in the early phase of AA*. The Qitai 110-m and Jingdong 120-m telescopes in China are also going to become operational over the next five years. Moreover, there are more stations available at Ku band in Oceania, Europe, and China. With 32 Gbps VLBI observations at Ku band, one could easily reach an image sensitivity of ~1 µJy/beam. This would allow astronomers to carry out some unprecedented research, such as detecting a ~10 µJy compact transient and gaining an astrometric precision of ~1 µas. Here, we show its scientific advantages and present some initial exploration results (status and future expansion) of the VLBI network at Ku band.
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VLBI observations with the SKA-Mid at 12/15 GHz Dr. Jun Yang Co-chair of SKA-VLBI Science Working Group Senior Research Engineer & VLBI Support Scientist Onsala Space Observatory, Chalmers University of Technology, Sweden The 10th International VLBI Technology Workshop, Chalmers, 21-25 Oct 2025, Sweden CHALMERS
Collaborators Jack Radcliffe (UK SKA Regional Centre) Cristina García Miró (IGN Yebes, Spain) Zsolt Paragi (JIVE, The Netherlands) Cormac Reynolds (ATNF, Australia) Bo Zhang (Shanghai Astronomical Observatory, China) Wen Chen (Yunnan Observatories, Chinese Academy of Sciences, China) Yingjie Li (Purple Mountain Observatory, China) 2
1. VLBI observing mode Imaging •continuum •spectral line Beamforming •pulsar search/timing •VLBI Transient buffer OBSERVING MODES External Correlator VLBI Control: VEX files. Output: multi-beam (<=4) and multithread VDIF data for VLBI correlation Note: Low-resolution and wide-field SKA Image cube files are also available for PIs. 1.1 Signal path
1.2 Sensitivity for the tied arrays System Equivalent Flux Densities (SEFDs) for phased-up VLBI beams Array SKA-Low (Core<1km) AA* Core: 199 st AA4 Core: 224 st SKA-Low (Inner 10 km) AA* : 271 st AA4 Core: 404 st SKA-Mid: inner 20 km diameter AA2 (64x15-m dishes, No MeerKAT dishes) AA* (77x15-m dishes at 0.3-15 GHz & 64x13.5 m dishes at 1.4 GHz) AA4 (110x15-m dishes at 0.3-15 GHz & 64x13.5 m dishes at 1.4 GHz) Band 50-350 MHz 1 (0.35-1.05 GHz) 2 (0.95-1.76 GHz) 5a (4.6-8.5 GHz) 5b (8.3-15.4 GHz) Freq. (GHz) 0.3 0.6 1.4 1.6 4.8 6.7 8.4 12.2 15.0 AA2 (Jy) No VLBI beam available 12.5 5.5 5.5 5.6 6.2 6.8 8.5 10.2 AA* (Jy) 18.3 13.5 6.3 2.7 4.4 4.6 5.1 5.6 7.0 8.5 AA4 (Jy) 16.3 9.0 4.6 2.2 3.1 2.7 3.0 3.3 4.1 4.9 4 Numerically, they are also the weighted continuum sensitivity (in uJy/beam) from the SKA Sensitivity Calculator using BW: 0.05 GHz, Time:10000 s, Natural weighting, EL: 45 deg, system efficiency: 0.9. References: (1) SKA-MID Sensitivity Calculator User Guide, (2) Anticipated SKA1 Science Performance
2. VLBI with the SKA-Mid at Ku band Paragi et al. 2015 Green et al. 2015 Conway et al. Memo 20-01 2.1 Unique advantages Providing the highest resolution (up to ~1 mas) for SKA-VLBI users to study innermost jets, cosmic transients, etc. (Paragi et al. 2015) The longest baseline: 10610 km between SKA-Mid and Tianma65. Having very wide observing bandwidth available for deep imaging observations up to 64 Gbps (8 GHz bandwidth per pol.) and powerful self-calibration on sources of a few mJy. VLBI image sensitivity is also dominated by the SKA-Mid and a few big dishes. Best observing band for the target sources close to the Galactic plane because of significantly less scattering broadening effect, i.e.𝜃scatter ∝ 𝜆2 and 𝜃obs ≈ 𝑠𝑞𝑟𝑡(𝜃2scatter + 𝜃2source), for point-like sources. Example: SKA-VLBI astrometry on Galactic radio stars or masers at 12.2 GHz (e.g. Green et al. 2015). Note that SKA Band 6 (15.3–50 GHz) receivers (Conway et al. Memo 20-01) are not funded for the SKA-Mid AA*. 5
62.2 Current status and future expansion Europe, Asia and Africa EVN and EAVN: Not available because of limited stations. VLBI stations available at Ku band Effelsberg (100 m), Tianma (65 m) & Yebes (40 m). Torun (32 m), Hartebeesthoek (25 m), Ibaraki (32 m) & Yamaguchi (32 m)at 12.2 GHz only. VGOS (VLBI Global Observing System) telescopes (13 m), e.g. Onsala Twin telescope. Upcoming stations Kunming (40 m): Installing a Ku-band receiver this year. Thai National Radio Telescope (TNRT 40 m): Installing a 4.6–13.7 GHz receiver in 2026. VLBI Exploration of Radio Astrometry (VERA): Developing 6-18 GHz receivers. MeerKAT (64 x 13 m dishes): Ordered 66 Band 5b receivers from MPIfR. Intend to develop Ku-band receivers: Sardinia (64 m), Medicina (32 m), & Onsala (20 m). Available new telescopes over next 5 years Qitai 110-m and Jingdong 120-m radio telescopes in China. Wetterstein Millimeter Telescope (WMT, 18 m), Botswana DZA and SKA-MPI dishes (15 m). Long Baseline Array (LBA) in the southern hemisphere Parkes (64 m): Started a 4-16 GHz receiver project and available in 2027+. Ceduna at 12.2 GHz. VLBA+VLA+GBT: Available and installing 8–40 GHz receivers for the VLBA.
7•Currently, 5 stations available at 12.2 GHz in the EVN. •~15 stations available for EVN+SKA-Mid observations at 12.2 GHz in 2030+. •More stations available in Japan, Australia. 10+ VGOS stations might be also included in the EVN, the EAVN, the LBA, and the SKA-VLBI observations. 2025 (ex VGOS and LBA stations) 2030+ Major concern: As the NRAO did, it requires an agreement with Starlink to get a radioquiet sky at 10.7–12.7 GHz.
3. Summary and outlook SKA-Mid and more new radio facilities would provide an unprecedented opportunity for us to perform the EVN and Global VLBI observations at Ku band. SKA-VLBI observations at Ku band allow astronomers to achieve high-sensitivity (~1 uJy/beam) and high-resolution (~1 mas) images, and extremely high astrometric precision (~1 uas). Welcome more stations to develop receivers at Ku band. Short VLBI tests will be organized to gain a complete view of the VLBI network performance at 12/15 GHz. 8 WMT Jingdong Qitai SKA-Mid Wang et al, 2022 Wang et al. 2023 Kadler et al. 2024