After the source pipeline completes for all detected sources in an observation interval (OBI), the master pipeline compares that OBI to others which overlap the same region of sky to match sources identified in multiple observations.
Match sources from different OBIs
The master pipeline cross-matches each detected source with all existing sources in the catalog database, and either inserts the derived source properties into the database in the case of a newly identified source, or merges the newly measured source properties with the existing catalog data in the case of an already cataloged source.
Source matches may be classified in several different ways:
New: the source is newly identified and doesn't overlap any OBIs, and so does not need to be merged.
Simple: all overlapping sources are unique within the OBI.
Confused: a single source in one OBI overlaps more than one source in another OBI.
Too hard: everything that doesn't fall into one of the other categories (e.g. "chained" sources).
Determine master source properties from the OBI entries
The properties for a merged source in the Master Catalog are determined from the individual source entries which were merged.
In most cases, the "best" value from contributing obervations is used, e.g. for spectral fit parameters (assuming pileup is not a factor). The Column Descriptions section outlines how each column value in the Master Catalog is derived from the individual source entries.
Package the data for inclusion in the CSC
All sources within a given observation are included in the Catalog as a unit, i.e. the full-field observations including those sources are linked together under each source name in the Master Catalog. This allows for straightforward updating and the ability to roll-back a single source if a problem is found.
Bidirectional links within the database record the connections between the master catalog entries and associated individual source entries, so that users can view all observations of a single source and how they contribute to the master entry.
Because the Chandra point spread function (PSF) is a strong function of off-axis angle, detections far off-axis in one observation may overlap multiple resolved sources detected on-axis in another observation. Such detections are flagged as "confused." The individual source entries are linked to the master catalog entries for each overlapped source, but the data are not used to compute the master source properties.
After this pipeline runs, sources are flagged for removal if the significance is lower than the threshold. When a source is removed, it is not included in the database.