# 14.5.9 panstarrs1_best_neighbour

Pan-STARRS1 BestNeighbour table lists each matched Gaia object with its best neighbour in the external catalogue.
There are $1\,327\,157$ objects in the filtered version of Pan-STARRS1 used to compute this cross-match that have too early epoch_mean.

Columns description:

source_id : Unique Gaia source identifier (long)

Unique identifier of the Gaia source, the attribute corresponds to gaia_source.source_id

original_ext_source_id : Original External Catalogue source identifier (long)

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 nearest neighbour in the External Catalogue

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.

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.