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gaia early data release 3 documentation

12.3 Crossmatches

12.3.14 urat1_best_neighbour

URAT-1 BestNeighbour table 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 URAT-1.
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 parameters covariance matrix of 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)

Columns description:

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 (float, Angle[arcsec])

Angular distance between a Gaia source and its best neighbour in the External Catalogue

xm_flag : Cross-match algorithm flag (short)

This flag is a bitmask indicating the details of the cross-match algorithm used for the source.

xm_flag values and descriptions:

  • 0 = Initial value; resets all bits.

  • 1 = The external catalogue object has one or more multiples. This means that there is at least another object with exactly the same astrometry.

  • 2 = The external catalogue object has one or more suspected duplicates. This means that there is at least another object much closer than the catalogue angular resolution.

  • 4 = The external catalogue object is resolved in Gaia.

  • 8 = The Gaia object has a five parameters astrometric solution.

  • 16 = The Gaia object has a two parameters astrometric solution.

  • 32 = The external catalogue object is matched only after the special treatment for sources with under-estimated position errors.

  • 64 = The external catalogue object is matched only after the special treatment for Gaia sources with large values of ipd_gof_harmonic_amplitude or ruwe.

For detailed documentation about xm_flag and the cross-match algorithm, see Chapter 8. For a detailed description of gaia_source columns ipd_gof_harmonic_amplitude and ruwe, see Chapter 4.

urat1_oid : External Catalogue source identifier (int)

The additional numeric unique source identifier of the External catalogue, increasing with Declination.

number_of_neighbours : Number of neighbours in External Catalogue (byte)

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 (byte)

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.