Niu Liu

Frame Tie in the Era of Multi-wavelength Celestial Reference Frames

Overview

The celestial reference frame (CRF) is fundamental to astronomy, geodesy, and deep-space navigation. The current fundamental celestial reference frame, the third realization of the International Celestial Reference Frame (ICRF3; Charlot et al. 2020), is based on the positions of extragalactic radio sources—mostly active galactic nuclei (AGNs)—measured by very long baseline interferometry (VLBI). With the advent of microarcsecond-level astrometry from both VLBI and Gaia, reference frames at different wavelengths must be linked with unprecedented accuracy.

An accurate celestial reference frame is essential for space-geodetic techniques and for measuring Earth orientation using VLBI, GNSS, DORIS, and satellite laser ranging. It also enables investigations of the Earth’s deep interior through Earth-rotation variability. In astronomy and astrometry, celestial reference frames support a wide range of applications, including tests of general relativity and Lorentz invariance, constraints on primordial gravitational waves, and measurements of Galactic rotation.

Reliable links between reference frames are also essential for interpreting observations obtained at different wavelengths. Comparisons of source positions across multiple bands provide a way to investigate the astrophysical properties of AGNs, identify strong gravitational-lensing candidates, and search for dual AGNs. Independent realizations of celestial reference frames at several wavelengths can also help distinguish catalog-dependent systematic errors from genuine physical signals.

Constructing a unified multi-wavelength celestial reference frame commonly assumes that the emission centers of AGNs measured at different frequencies coincide. However, this assumption does not always hold. Possible causes include technique-dependent biases, frame-alignment errors, large-scale deformations of the celestial frame, radio core shifts, optical and radio source-structure effects, host-galaxy contamination, photometric variability, strong gravitational lensing, and dual AGNs.

This project investigates the systematic and astrophysical effects that limit the accuracy of multi-wavelength frame ties. It combines studies of large-scale systematics, core shifts, and source-structure effects with the selection of suitable sources whose positions are less affected by these phenomena.

Objectives

Funding

  1. Liu N., Lambert S. B., Arias E. F., Liu J.-C., & Zhu Z. Evaluation of the ICRF stability from a position time series analysis. A&A 659, A75 (2022).

  2. Liu N., Lambert S. B., Charlot P., Zhu Z., Liu J.-C., Jiang N., Wan X.-S., & Ding C.-Y. Comparison of multifrequency positions of extragalactic sources from ICRF3 and Gaia EDR3. A&A 652, A87 (2021).