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Last modified: 28 August 2013

Why isn't my source included in the catalog?

There can be several reasons for why a particular source does not appear in Release 1 of the Chandra Source Catalog:

  • The CSC is not an all-sky catalog; it is constructed from pointed observations, and does not include sources detected to a uniform depth. Furthermore, there are many filters that determine which data sets are included or excluded from each release of the catalog; refer to the Observation Selection page for details on Release 1.
  • The first release of the catalog includes only point and compact sources, with observed (i.e., un-deconvolved) spatial extents <~30 arcsec; sources larger than this will not be detected with the current CSC algorithms. In addition, observations of fields containing extended sources have been excluded from the catalog, or in some cases only a part of the field has been included. A list of ObsIDs with dropped chips is available here.
  • The source could have failed to satisfy catalog quality assurance (QA) and catalog inclusion (CI) filters in processing; in particular, the CI filter imposes a source significance threshold. For more information, refer to the CSC memo Chandra Source Catalog Quality Assurance Specifications.
  • The source could have been too faint for detection. Limiting sensitivity maps for all observations contained in the CSC are available for download via CSCview.
  • When compiling the master source lists, sources from multiple overlapping observations are merged together. This means that a source which appears to be singular in one off-axis OBI may in fact be resolved into multiple sources in an overlapping, on-axis OBI. In this case, the apparently singular source in the off-axis OBI is considered a confused source, and therefore is ambiguously linked to each of the resolved master sources in the on-axis OBI (as indicated by 'match_type=a'). As a result, the confused source would not be located in a search on master source properties in CSCview, since the properties of confused sources are not used in the calculation of CSC master source properties.