APASS DR9 Best Neighbour

Table <code>apassdr9_best_neighbour</code> lists each matched Gaia object with its best neighbour in the external catalogue. The cross-match algorithm is not symmetric and searches Gaia sources counterparts in APASS DR9. The best neighbour is chosen among good neighbours as the one with the highest value of the figure of merit, which evaluates the ratio between two opposite models/hypotheses: the counterpart candidate is a match or it is found by chance. Good neighbours are nearby objects in the external catalogue whose position is compatible within position errors with the Gaia target. The cross-match algorithm is positional and exploits the full 5 parameter covariance matrix of the Gaia astrometric solution when available and the external catalogue positions and position errors. In addition it takes into account the external catalogue environment using the local density.
Please note that the cross-match algorithm is a trade-off between multiple requirements, in particular between completeness and correctness. It is thus not limited to a simple cone search.
Reference papers:Marrese et al. (2017)Marrese et al. (2019)