PPMXL BestNeighbour table lists each matched Gaia object with its best neighbour in
the external catalogue.
source_id : Unique Gaia source identifier (long)
A unique single numerical identifier of the source obtained from gaia_source (for a detailed description see gaia_source.source_id)
original_ext_source_id : Original External Catalogue source identifier (string)
The unique source identifier in the original External catalogue.
angular_distance : Angular Distance between the two sources (double, Angle[arcsec])
Angular distance between a Gaia source and its best neighbour in the External Catalogue
gaia_astrometric_params : Number of Gaia astrometric params used (short)
This field indicates the number of Gaia astrometric parameters which were available in Gaia.
The field is set to 2 when only RA and DEC where available, while is set to 5 when RA, DEC,
PMRA, PMDEC and PARALLAX are available and thus used to propagate a Gaia source position to the External Catalogue source coordinates epoch.
The additional numeric unique source identifier of the External catalogue, increasing with Declination.
number_of_neighbours : Number of neighbours in External Catalogue (int)
Number of sources in the External Catalogue which match the Gaia source within position errors.
The identifiers of all the neighbours can be found in the Neighbourhood table.
number_of_mates : Number of mates in Gaia Catalogue (short)
Number of other Gaia sources that have as best-neighbour the same External Catalogue source.
In case there are no other Gaia sources with the same best-neighbour in the external catalogue, the number of mates is equal to zero.
Given the Gaia high angular resolution, it will happen that what appears as a single object in an external catalogue will be resolved by Gaia and as such will be the best-match of more than one Gaia object.
best_neighbour_multiplicity : Number of neighbours with same probability as best neighbour (short)
The best-match to a Gaia source in an external catalogue is the source in the external catalogue that has the highest probability to be the best-match.
As the probability is based on positional and density properties, it could happen that there is more than one source in the external catalogue with the same probability.
Even if a single best-match is always chosen, this field tells the user if there were more ”best” neighbours. Those neighbours can be found in the Neighbourhood table.