Niu Liu

Project: 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 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, we are entering an era in which reference frames at different wavelengths and frequencies must be linked with unprecedented accuracy. This project investigates the systematic effects that limit such frame ties and explores methods for constructing a unified multi-wavelength celestial reference frame.

An accurate celestial reference frame is crucial for space-geodetic techniques and for measuring Earth orientation with VLBI and other satellite techniques, including GNSS, DORIS, and SLR. It also supports studies of the Earth’s deep interior based on Earth-rotation variability (Buffett 1992; Buffett et al. 1993; Mathews et al. 2002). In astronomy and astrometry, celestial reference frames have broad applications, including tests of fundamental physics, such as general relativity and Lorentz invariance, the estimation of the amplitude of primordial gravitational waves, and the measurement of Galactic rotation.

In the era of multi-wavelength celestial reference frames, a reliable and consistent frame is essential for interpreting observations made at different wavelengths. Comparisons of source positions across several bands also provide a new way to study the astrophysical properties of AGNs, search for strong gravitational-lensing candidates, and identify dual AGNs. More broadly, independent realizations of CRFs at different wavelengths make it possible to separate systematic errors from other physical signals, which cannot be achieved by comparing only two catalogs.

Celestial reference frames based on observations at different frequencies and wavelengths need to be linked together to construct a unified multi-wavelength celestial reference frame. In this process, the emission centers of AGNs measured at different frequencies are often assumed to coincide. However, this assumption does not always hold. Possible causes include technique-dependent biases, frame alignment errors, deformations of the celestial frame, radio core shift, source-structure effects in both optical and radio measurements, host-galaxy contamination, photometric variability, strong gravitational lensing, and dual AGNs. These effects need to be mitigated to achieve a reliable link between celestial reference frames, especially the frame tie between the Gaia-CRF and the ICRF.

This project aims to improve the accuracy of multi-wavelength frame ties in two complementary ways. First, large-scale systematics and astrophysical effects, including core shift and source structure, are investigated and modeled where possible. Second, the frame tie is optimized using an ensemble of carefully selected sources whose positions are less affected by these effects. The project focuses on the following topics:

Objectives

Funding

China Postdoctoral Science Foundation

  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. Astronomy & Astrophysics, 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. Astronomy & Astrophysics, 652, A87, 2021.