Project: Investigation of frame-tie in the era of multiwavelength celestial reference frame

Investigation of frame-tie in the era of multiwavelength celestial reference frame

Descriptions

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The celestial reference frame (CRF) is a cornerstone of several scientific domains in the astronomy, geodesy, and deep space mission. The current fundamental reference frame is the third generation of International Celestial Reference Frame (ICRF3, Charlot et al. 2020), which is consisted of positions of extra-galactic objects (mostly Active Galactic Nucleus (AGNs)) measured by the very long baseline interferometry (VLBI) at S/X-, K-, X/Ka-band (8, 24, and 32 GHz, respectively). Although these three components are all obtained by VLBI, they can be considered as independent since they use different networks and observational strategies. At the optical domain, the Gaia mission provides an independent realization of reference frame, Gaia-CRF2 (Gaia Collaboration et al. 2016, 2018), with an accuracy comparable to the ICRF3. We are stepping, for the first time of modern history, into an era of multi-wavelength celestial reference frame, in the sense of independent celestial frame realizations with comparable accuracies of microarcsecond level.

An accurate celestial reference frame is crucial for space geodetic techniques and in the measurement of Earth orientation by VLBI and other satellite techniques (GNSS, DORIS, SLR) and serves in Earth’s deep interior studies based on Earth rotation variability (Bffett 1992; Buffett et al. 1993; Mathews et al. 2002). It has also strong applications in astrometry and astronomy including tests of fundamental physics (General relativity: Lambert & Le Poncin-Lafitte (2009, 2011), Titov et al. (2018); Lorentz invariance: Le Poncin-Lafitte et al. (2016)), the estimate of the amplitude of primordial gravitational waves: Gwinn et al. (1997); Titov et al. (2011)), measurement of the Galactic rotation (e.g., Titov et al. 2011; Titov & Lambert 2013; MacMillan et al. 2019). The United Nations Committee of Experts on the Global Geospatial Information Management (UN-GGIM) has already recognized the importance of the reference frame and established a resolution to encourage studies and developments of a Global Geodetic Reference Frame, including the ICRF and International Terrestrial Reference Frame (ITRF).

In the era of multi-wavelength celestial frame, a reliable and consistent celestial reference frame enables interpreting the observations at different wavelengths correctly. Besides, the comparison between positions at several bands opens a new door to understand the astrophysical properties of AGNs (e.g., Plavin et al. 2019) and search for strong gravitational lensing candidates and dual AGNs (e.g., Orosz & Frey 2013). In a global sense, the co-existing independent realization of CRFs at different wavelengths allows separating systematical error from other signals, which cannot be done by inter-comparison between merely two catalogs (e.g., Liu et al. 2019). The celestial reference frames based on observations at different frequencies need to be linked together and construct a multi-wavelength celestial reference frame. In this process, the centers of emission of AGNs measured at different frequencies are usually assumed to coincide (e.g., Karbon & Nothnagel 2019). However, this assumption does not always hold due to several reasons. The first one is the technique-dependent bias, for instance, alignment and deformations in the celestial frame (Liu et al. 2019). The core-shift and source structure effect on both optical and radio measurements would also result in an offset of emission centers (Kovalev et al. 2008; Porcas 2009; Petrov & Kovalev 2017a). Other cases include the existence of host galaxy, photometric variability, strong gravitational lensing, and dual AGNs (Bachchan et al. 2016; Frouard et al. 2018; Makarov et al. 2019). These effect needs to be mitigated to permit a reliable link of celestial frames, especially the frame-tie between the Gaia-CRF and ICRF, which is also a non-trivial task for the Gaia mission.

The efforts to improve the frame-tie accuracy could be made in two ways simultaneously. On the one hand, the large-scale systematics and astrophysical effects (core-shift effect and source structure) could be studied and then modeled, if possible, before the frame-tie. On the other hand, the frame-tie could be carried out via an ensemble of well-selected sources whose position is less affected by the effects mentioned above. The proposed project aims to address several problems related to the frame-tie of the multi-wavelength reference frame, using the next data releases of Gaia, i.e., Gaia EDR3 and DR3, as well as the up-to-date VLBI solutions. These problems include

• (i) examination of large-scale errors in the reference frames;
• (ii) selection of suitable AGNs and optimization of the frame tie;
• (iii) understanding relation of the absolute position and frequency for AGNs.

Funding

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China Postdoctoral Science Foundation

Keywords

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• Celestial reference system (frame)
• Extragalactic reference frame
• Frame-tie

Research Output

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