Caveats and Limitations

Catalog Data

General

In particular, users of the CSC should be aware that there may be fundamental and significant selection effects that restrict the source content of the catalog and which therefore may limit scientific studies that require an unbiased source sample. The CSC is constructed from pointed observations obtained using the Chandra X-ray Observatory; it is not an all-sky catalog, and does not include sources detected to a uniform depth. Users should also be aware that the detection of sources with spatial extent more than ~30 arcsec is done with a separate algorithm from that for compact sources (the sources have names which end in an 'X' to distinguish them from compact sources).

Note that for some sources in observations with short exposures, there are zero counts in the local background region for some bands. In these cases, the source flux is not computed.

Specific Caveats and Limitations applicable through CSC release 2.1.1

• The true astrometric position error should be consistent with the reported position error for ~90% of the sources. For the ~10% of sources for which this is not the case are not distributed randomly. There are a small number of stacked observations for which the absolute error may be significantly larger than reported (up to a maximum of ~2.0 arcseconds).
• The model independent flux_aper values are computed from the events' observed energy. In narrow energy bands, especially the ultra-soft (0.3-0.5keV) band, the spectral response can change rapidly with energy. This means a small change in observed energy from an event's true energy (as described by the RMF) can result in large changes in the flux. If there are few events this can result in large, undetermined systematic errors. In the other energy bands where spectral response generally changes slowly with energy this is less of a problem. It also becomes less of a problem when there are a large number of events as the Gaussian-like RMF is well sampled and the uncertainty balances out.
• Some data products such as PHA files (pha3) contain 'pixel masks'. Users of the Chandra Interactive Analysis of Observations (CIAO) data analysis package prior to version 4.10 may encounter problems using these files with some tools.
• Users of the XSPEC X-ray spectral fitting package may need to manually split the source region PHA spectrum file (pha3) into separate source region and background region spectra prior to analysis in XSPEC; see the page "Using the L3 Data Products" for instruction. Users of Sherpa can just read in the file with load_pha or load_data, which will read in the background automatically, and so do not need to split up the file.

Specific Caveats and Limitations for certain ObsIDs

• Some ACIS observations were performed with energy filters applied. Depending on the particular observation, this could remove all or most of the sensitivity for certain CSC energy bands. These data will have NULL values for some of the flux properties and possibly the hardness ratios. More details can be found in the ACIS event energy threshold caveat.

  This caveat also applies to the properties calculated for the convex-hull sources, athough in this case only fluxes are calculated (no spectral fits, hardness ratios, or timing analysis), so the number of affected columns is less than for compact sources.

Specific Caveats and Limitations applicable through CSC release 2.1

Incorrect Source Names in CSC 2.1

Missing Source and Detection Properties

We have identified a number of cases (~7000 master sources) where master source and stacked observation detection properties (primarily aperture photometry properties and properties derived from them, such as hardness ratios and temporal variability) may be incorrectly set to NULL in some energy bands. This occurs for master sources and stacked observation detections for which one or more ACIS observations has energy bands truncated by onboard PHA filters. In such cases, the per-observation detection properties for the impacted energy bands should be NULL, but master source and stacked observation detection properties should still be populated with non-NULL values if there are additional observations that do not have PHA filters applied. The majority of impacted sources are located in the vicinity of Sgr A*.

This issue was fixed in CSC release 2.1.1.

Issues for users of the preliminary release of CSC 2.1

Note

Version 2.1 of the Chandra Source Catalog has now been finalised, as of 2^nd April 2024. Prior to this date the CSC 2.1 data was made available to the community as the "current database" in CSCview. The following section is only relevant for people who used the CSC 2.1 data before this release.

The "current database" view of the catalog changed quasi-continuously over time as datasets were processed. It is strongly suggested that queries are repeated against the finalised 2.1 release.

Catalog Interfaces

Access to a limited set of data products related to convex-hull sources will be made available soon. The affected products are the master-source poly3 files and the source images.

CSCview

The software-requirements page contains current instructions for downloading and running the CSCview java application.

Interoperability with other tools

• Users are warned that there is a limit to the amount of data which may be imported into CSCview from a remote, SAMP-connected client (such as Topcat), using the crossmatch User table --> Received Table option in order to perform a crossmatch of this data against the CSC. This limit is 250000 rows in a table of data being imported, e.g., a table containing a full catalog of astronomical data to be crossmatched against the CSC. This means that the number of source matches reported by CSCview will be incorrect, as only the first 250000 rows of the input table will have been used in the query. It is important to note that when an incoming table of data is truncated by CSCview, no warning is issued.

CSCweb

The quick-search web page allows you to search for a fixed set of master-source properties, and the limiting sensitivity, for one or more positions on the sky. The limiting-sensitivity search is limited in the radius over which it will search.

The contributed scripts

Please ensure that you are using the latest version of the CIAO contributed-scripts package.