How Well is the International Celestial Reference System Maintained in Official IAU Implementations?

Abstract

The International Celestial Reference Frame (ICRF) based on the VLBI-derived positions of 608 extragalactic radio sources was adopted by the International Astronomical Union (IAU) in 1998 as the first realization of the International Celestial Reference System (ICRS). Later, in 2009 and 2020, two extended ICRF versions, ICRF2 and ICRF3, respectively, were released. The latter is adopted by the IAU as the current implementation of the ICRF in the radio band. In the meantime, the Gaia mission delivered three versions of the optical ICRS realization in 2016, 2018, and 2022 with an accuracy similar to that achieved by VLBI-based ICRF. The Gaia-CRF catalogs were linked to the ICRF under no-rotation conditions and thus may suffer from ICRF systematic instability if the latter is substantial. In this work, a new analysis was performed to assess the long-term stability of radio and optical ICRS realizations. Based on the 16-parameter vector spherical harmonics expansion of the differences between the three ICRF catalogs, it can be concluded that the mutual orientation between them is at a level of a few tens of microarcseconds, while the components of the glide vector and E 2,0 term are several times greater. A comparison of the three Gaia-CRF catalogs with the ICRF3-SX showed that for the latest Gaia-CRF catalog, Gaia-CRF3, all rotational and deformation components are below 20 μas except for the E 2,0 term, which is several times greater. For both ICRF and Gaia-CRF catalogs, the evolution of the source position errors is also tracked.