4.5.3 Source verification

The ranges and distributions of the astrometric parameters and various statistics were monitored as part of the internal validation performed by the AGIS team.

Residuals of the Gaia DR2 solution have been monitored during the iterative solution and extensively studied as part of the internal validation performed by the AGIS team. In particular the internal consistency checks were done to assess the level of systematic errors in the solution. The overall level of residuals is of the order of 0.15 mas for $G\lesssim 13$ to several mas for faint sources (Figure 4.20). The systematic part of the residual (median residual as a function of magnitude, colour, AC rate, and similar) is typically on the level of 0.01–0.03 mas (Figure 4.21). See Appendix A.1–A.3 of the Gaia EDR3 astrometry paper (Lindegren et al. 2021) for a more complete discussion of the results.

The internal consistency of the astrometric solution can be examined by comparing solutions based on complementary subsets of the observations. The observations can for example be divided depending on the CCD strip in the astrometric field (AF). Normally a source is observed in nine consecutive CCD strips, denoted AF1–AF9. Separate solutions have been obtained using observations on AF2–AF5 and AF6–AF9, respectively; these are called the ‘early’ and ‘late’ solutions. The split-field solutions show an overall offset in the parallaxes of some 20–30 $\mu$as between the two solutions, largely coming from the difference in the global parameter $\delta C_{1,0}$ (Section 4.3.7); but more importantly the solutions show the presence of magnitude- and colour-dependent systematic errors, mainly affecting the bright ($G$ $\lesssim 13$) sources (Figure 4.22), and spatial variations on angular scales from below one degree to tens of degrees (Figure 4.23). These differences probably give a good impression of the sizes and general character of the actual astrometric errors in Gaia EDR3.

The reference frame of Gaia EDR3 is nominally aligned with ICRS and non-rotating with respect to the distant universe. This was achieved by means of a subset of 429 249 primary sources assumed to be quasars, including 2269 sources identified as the optical counterparts of VLBI sources in ICRF3-SX (Charlot et al., 2020, forthcoming). The alignment procedure used the frame rotator described in Section 4.3.2. In the final AGIS solution the frame rotator rejected some of these sources as outliers; in the end 428 034 of the sources were used to fix the spin of the reference frame and 2007 were used to fix the orientation. The list of sources considered and used by the frame rotator is available in Gaia EDR3 as the table frame_rotator_source.

A careful cross-matching of Gaia EDR3 with various quasar catalogues has resulted in a much larger set of sources identified as quasars (active galactic nuclei, AGNs). The positions and proper motions of the sources in this list, available in Gaia EDR3 as the table agn_cross_id, provides the actual definition of the celestial reference frame of Gaia EDR3, known as Gaia-CRF3. Its properties are extensively discussed in Klioner et al. (2020) and Gaia Collaboration et al. (2020b).