Gaia data processing cycles start with a process that groups individual Gaia observations and links them to sources on the sky. This leads to a working catalogue of sources (hereafter called ‘the source list’) and their corresponding observations, which forms the basis for the subsequent data processing. The algorithm that carries out the grouping and linking has been much improved at the beginning of the current processing cycle and this has led to many changes in these groups, resulting in a significant fraction of the so-called source identifiers at the bright end (G<12) of the internal working catalogue being changed. Given the Gaia data processing flow this would severely limit the contents of Gaia DR2. In consultation with the Gaia Science Team and the DPAC Executive it was decided that this is not acceptable.
Consequently, the DPAC project office, together with ESA, has worked on consolidating a revised schedule for Gaia DR2. A more self-consistent processing flow was designed which ensures that all DPAC pipelines base their processing on the same input source list. The revised schedule was extensively discussed with all involved and the new release date for Gaia DR2 will be April 2018.
The potential contents of Gaia DR2 are detailed here. It is important to mention that during the revision of the schedule optimisations were introduced that not only improve the internal consistency of the data processing, but also allow for more data products than originally foreseen to be included in Gaia DR2.
Users of Gaia data should be aware that the source list for Gaia DR2 should be treated as independent from Gaia DR1. Although the majority of sources in Gaia DR1 can be identified with the same source in Gaia DR2 through the Gaia source identifier, the improved source list will lead to the following changes in linking the observations to the source identifiers for a substantial fraction of sources in this list:
- The merging of groups of observations previously linked to more than one source will lead to a new source associated to the merged observations (with a new source identifier) and the disappearance of the original sources (along with their source identifiers).
- The splitting of groups of observations previously linked to one source will lead to new sources associated to the split groups of observations (with new source identifiers) and the disappearance of the original source (along with its source identifier).
- The list of observations linked to a source may change (and hence the source characteristics may change), while the source identifier remains the same.
A means to trace sources across the Gaia DR2 and Gaia DR1 source lists will be provided.
The DPAC project office and ESA are also working on the longer term data release schedule. The release planned at three years after the end of the nominal mission lifetime (called 'final release' in the data release scenario) will be maintained, while the number of releases between Gaia DR2 and the final release remains to be decided. Note: the total astrophysical content promised to be released will not be changed.