Chandra Source Catalog

Ian Evans

CSC Release 2.0

Release 2.0 of the Chandra Source Catalog (CSC 2.0) was made available to the community on October 24, 2019. CSC 2.0 is the second major release of the catalog and represents a significant improvement in terms of sky coverage, sensitivity and capabilities with respect to previous versions. By combining Chandra’s sub-arcsecond on-axis spatial resolution and low instrumental background with consistent data processing, the CSC delivers a wide variety of uniformly calibrated properties and science ready data products for detected sources over four decades of X-ray flux.

The catalog includes measured and derived properties for 317,167 unique compact and extended X-ray sources in the sky, allowing statistical analysis of large samples, as well as individual source studies. Extracted properties are provided for 928,280 individual observation detections (1,420,545 including photometric upper limits) identified in 10,382 Chandra ACIS and HRC-I imaging observations released publicly through the end of 2014. Photometric properties for 1,299 highly extended (≳30) sources are provided, together with surface brightness polygons for several contour levels. The total cumulative sky coverage of CSC 2.0 is 558.65 deg², and multi-band limiting sensitivity is computed for the entire sky coverage of the catalog at a resolution of 3.22 × 3.22 arcsec. A comparison of CSC 2.0 with several other X-ray mission catalogs is shown in Figure 1.

The sensitivity limit for compact sources in CSC 2.0 is ~5 net counts (on-axis) for most observations, which is a factor of ≳2 better than the previous catalog release. This improvement is achieved by using a two-stage approach that involves co-adding multiple observations of the same field prior to source detection, and then using an optimized source detection approach that combines multiple algorithms to identify candidate detections and a maximum likelihood estimator to grade those candidate detections.

For each X-ray detection and source, the catalog provides a detailed set of more than 100 tabulated positional, spatial, photometric, spectral, and temporal properties (each with associated lower and upper confidence limits) measured in 5 energy bands for ACIS and a single energy band for HRC-I. As a result of this multiplexing, the catalog databases include approximately 1700 columns of information, split across several tables. A Bayesian aperture photometry code produces robust photometric probability density functions (PDFs), even in crowded fields and for low count detections. Release 2.0 uses a Bayesian Blocks analysis to identify multiple observations of the same source that have similar multi-band photometric properties, and these are analyzed simultaneously to improve S/N.

The catalog includes roughly 40 different types of science-ready source- and field-based data products, totaling almost 36 TB, including photon event-lists, multi-band images, backgrounds, exposure maps, and PSFs, position error MCMC draws, extended source contours, aperture photometry probability density functions (PDFs), Bayesian Blocks properties, spectra, instrument responses, and light-curves. Since these data products are pre-computed by applying all of the appropriate calibration steps (e.g., matching astrometry, merging observations, applying exposure corrections, removing background) included in the catalog pipelines, they can be used directly by the end-user to simplify significantly the effort required to perform detailed scientific analyses of properties for scientifically meaningful samples of sources without manipulating large volumes of data.

Data access and documentation for the catalog are available through the catalog web site. The catalog documentation describes the content and organization of the catalog data, the data processing steps, and includes detailed descriptions of the tabulated source and detection properties and data products. The Caveats and Limitations page provides important information about the catalog that should be reviewed by all users prior to using the catalog data.

Multiple user-interfaces to the catalog are accessible via the catalog web site. View in WWT provides a visual interface based on the American Astronomical Society’s World Wide Telescope that exposes the outlines of the stacked-observations as well as the locations of the catalog sources, and allows the user to explore the sky coverage and content of the catalog.

Clicking on a source brings up a box with a basic set of source properties. The Quick Search web interface supports position-based and crossmatch searches and returns a limited set of source properties, making it ideal for simple queries such as determining whether a source detected in another waveband has a Chandra X-ray counterpart. Virtual Observatory tools that support the IVOA Simple Cone Search (SCS), Table Access Protocol (TAP), and Simple Image Access Protocol (SIAP) can access the corresponding catalog interfaces directly. Web forms that allow queries via these interfaces are also provided. For more sophisticated queries and to provide access to the full functionality of the catalog, we recommend using the downloadable CSCview data mining application. This interface allows arbitrary sets of tabulated properties to be retrieved based on combinations of user-specified constraints on any set of properties plus positional searches or crossmatches and returns tabulated results that may be saved to a file or SAMP’ed to another application. Catalog queries can be entered via a simple forms-based interface or alternatively via the ADQL query language. CSCview also provides options to retrieve any desired science-ready FITS data products. The CIAO data analysis package includes tools to search the catalog via the catalog command-line interface. Finally, a subset of the CSC 2.0 data (typically the master sources table, but not the stacked observation detections table, the per-observation detections table, or the FITS data products) are accessible through multiple external services, including HEASARC, CDS Vizier, NED, and ESASky.

Future Catalog Releases

An issue was recently identified in the CSC 2.0 master sources table for sources detected in multiple observations that were distributed across more than one Bayesian Block. Under these circumstances, only a subset of observations that included a source were considered when computing observation variability properties (temporal and spectral variability), likely resulting in source variability being underestimated. In addition, the calculation of inter-observation variability in cases where one or more of the input observations has a flux upper limit (or a hardness ratio lower or upper limit for spectral variability) could be incorrect. The CXC is working on an update to the catalog, release 2.0.1, to correct these issues. CSC 2.0.1 is expected to be released publicly prior to the end of the year.

The next release of the catalog, CSC 2.1, will augment CSC 2.0 with data from observations released publicly since the end of 2014. While intended primarily to be a data “catch-up” release, CSC 2.1 will include a few updates. Most of these updates are aimed at improving the robustness of existing catalog algorithms and reducing the amount of manual effort required to build the catalog, while not introducing changes that might impact the statistical properties of the catalog. From the end-user perspective, the most important update will be to tie catalog astrometry to the Gaia reference frame. This change will be applied uniformly to all sources, including those otherwise unchanged from CSC 2.0. While the Right Ascensions and Declinations of sources and detections may change by a small amount (typically th bulk reprocessing of Chandra data completes (expected in late 2021) and will require approximately 1 year to complete.