Gaia Data Release 2: The Celestial reference frame (Gaia-CRF2)
F. Mignard, S. Klioner, L. Lindegren, J. Hernandez, U. Bastian, A., Bombrun

TL;DR
Gaia DR2 provides a highly accurate, dense optical reference frame based on over half a million quasars, aligning with ICRS standards and supporting precise celestial positioning.
Contribution
This paper introduces Gaia-CRF2, the first non-rotating optical celestial reference frame built solely on extragalactic sources, meeting ICRS criteria.
Findings
Median positional uncertainty of 0.12 mas for bright sources
Good overall agreement with radio positions from ICRF3
Systematic errors estimated at 20-30 microarcseconds
Abstract
The second release of Gaia data (Gaia DR2) contains the astrometric parameters for more than half a million quasars. This set defines a kinematically non-rotating reference frame in the optical domain referred to as the Gaia-CRF2. The Gaia-CRF2 is the first realisation of a non-rotating global optical reference frame that meets the ICRS prescriptions, meaning that it is built only on extragalactic sources. It consists of the positions of a sample of 556 869 sources in Gaia DR2, obtained from a positional cross-match with the ICRF3-prototype and AllWISE AGN catalogues. The sample constitutes a clean, dense, and homogeneous set of extragalactic point sources in the magnitude range G from 16 to 21 mag with accurately known optical positions. The median positional uncertainty is 0.12 mas for G < 18 mag and 0.5 mas at G = 20 mag. Large-scale systematics are estimated to be in the range 20 to…
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