Applications of the Source-Frequency Phase-Referencing Technique for ngEHT Observations

Abstract

The source-frequency phase-referencing (SFPR) technique has been demonstrated to have great advantages for mm-VLBI observations. By implementing simultaneous multi-frequency receiving systems on the next-generation Event Horizon Telescope (ngEHT) antennas, it is feasible to carry out a frequency phase transfer (FPT) which could calibrate the non-dispersive propagation errors and significantly increase the phase coherence in the visibility data. Such an increase offers an efficient approach for a weak source or structure detection. The SFPR also makes it possible for high-precision astrometry, including the core-shift measurements up to sub-mm wavelengths for Sgr A*, M 87*, etc. We also briefly discuss the technical and scheduling considerations for future SFPR observations with the ngEHT.

Key Science Goals for the Next-Generation Event Horizon Telescope

“…The multi-band capabilities of the ngEHT will enable the use of the source-frequency phase referencing ([SFPR; [199]) technique, potentially achieving ∼µas-level astrometry for targets that are sufficiently bright and close to known reference sources [46,47]. In addition to many other scientific applications such as measurements of (chromatic) AGN jet core shifts (e.g., [200][201][202]) and the (achromatic) orbital motions of binary SMBH systems (e.g., [141]), one of the opportunities afforded by this astrometric precision is a measurement of the so-called "cosmological proper motion" [203] or "secular extragalactic parallax" [204].…”

“…The next-generation EHT (ngEHT) is a project to design and build a significantly enhanced EHT array through the upgrade, integration, or deployment of additional stations (e.g., [37][38][39][40][41][42][43][44]), the use of simultaneous observations at three observing frequencies [45][46][47], and observations that extend over several months or more with a dense coverage in time [48]. The ngEHT currently envisions two primary development phases.…”

Enabling Transformational ngEHT Science via the Inclusion of 86 GHz Capabilities

“…Finally, an integer frequency ratio is preferred between the different frequency bands in order to optimally use these techniques and should allow bonafide astrometry between the bands (i.e., between 86 and 230 and/or 345 GHz). Astrometry at these frequencies is expected to yield residual systematic errors of ∼3 µas [27,[37][38][39], though we note that such analyses will additionally need to account for frequency-dependent structural changes such as the well-known core-shift effect (e.g., [40]). These specifications are discussed in more detail in Section 3.4.…”

Sub-mm spectral astrometric VLBI with the ngEHT

“…This was focused on the lower frequencies and SKA, but the conclusions are still applicable to the higher frequencies, and potentially ngEHT. For example we have completed a study of the expected performance of the Frequency Phase Transfer (FPT) between the 86-and 340-GHz frequency bands (Rioja et al (2023)), and a survey of the major science applications (Jiang et al (2023)). Figure 2, reproduced from Rioja & Dodson (2020), shows schematically the possible station combinations for sub-mm astrometric VLBI.…”