Users wishing to look up the DR2 record for an astrophysical source identified in DR1 must NOT simply extract the record from DR2 having the same source identifier.
As described in the detailed description of attribute designation in gaia_source it is not guaranteed that the same astronomical source will always have the same source identifier in different Data Releases. Hence the only safe way to compare source records between different Data Releases in general is to check the records of proximal source(s) in the same small part of the sky. This table provides the means to do this via a precomputed crossmatch of such sources, taking into account the proper motions available at DR2.
Within the neighbourhood of a given DR2 source there may be none, one or (rarely) several possible counterparts in DR1 indicated by rows in this table. This occasional source confusion was introduced during the DR1 processing which used an earlier version of the software for matching of transit observations to unique astrophysical sources. The subsequent merging, splitting and deletion of identifiers introduced at DR1 during the DR2 processing means there is no guaranteed one–to–one correspondence in source identifiers between the releases.
For more details of the procedure used to create this crossmatch, see Chapter 11 in the online documentation.
dr1_source_id : Source identifier in Gaia DR1 (long)
Source identifier assigned during DR1 processing for an astrophysical object in close proximity to (possibly) the same object in DR2.
dr2_source_id : Source identifier in Gaia DR2 (long)
Source identifier assigned during DR2 processing for an astrophysical object in close proximity to (possibly) the same object in DR1.
angular_distance : Angular distance between the two sources (float, Angle[arcsec])
Angular (great–circle) distance on sky between the source pairing between DRs 1 and 2.
magnitude_difference : G band magnitude difference between the sources (float, Magnitude[mag])
G band magnitude difference between the sources based on the individual magnitudes materialised in gaia_source at the time of the respective data releases. The sense of the difference is DR2 minus DR1.
rank : Rank metric indicating relative probability of a good match (float)
Probability figure–of–merit giving some indication of how probable is the association between these two sources in terms of the typical distribution of the position and magnitude differences for the crossmatch as a whole. For more details see Chapter 11 in the online documentation.