Caveats and Limitations

While the Chandra Source Catalog (CSC) is capable of supporting many diverse scientific investigations, users should carefully consider the limitations of the CSC when assessing the efficacy of the catalog for their particular line of enquiry.

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. Furthermore, 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 highly extended sources have been excluded from the catalog. In most cases if the extended emission is restricted to a single ACIS CCD, then the data from the remaining CCDs is included in the catalog.

Users of catalog database views (as opposed to an official catalog release view) should also be aware that the contents of the database are subject to revision as processing progresses, and a small fraction of sources present in the catalog database may not meet all of the catalog release quality assurance requirements.

Specific Caveats and Limitations applicable through CSC release 1

• In release 1.0 of the catalog, the var_flag property in the Master Sources Table was not correctly populated. This error is fixed in release 1.0.1. Catalog users who extracted this property should consider re-running their queries against the new release.

• We have identified an error in variability determination for a subset of sources near chip edges. This will affect sources that dither across multiple CCDs, where one or more of the CCDs are off or are otherwise excluded. Events from the off/excluded CCDs are not included in the source lightcurve nor in any other source properties; however, the fractional area correction (calculated with the dither_region tool) will still include areas from the off/excluded CCDs when applying the variability tests. This can lead to a false variability signal from all three variability tests (Kuiper, Kolmogorov-Smirnov, and Gregory-Loredo). This variability will not necessarily be on the dither time scale, therefore the dither_warning_flag may be set to FALSE and thus not serve as an indicator of this problem. Likewise, the multi_chip_code flag may be set to 0 if all but one CCD is off/excluded. Thus, highly variable sources with a non-zero edge_code should be treated with caution when assessing variability properties.

• The background is modeled using a single low spatial frequency component, with the addition of a high spatial frequency "streak map" for ACIS observations. The detectability of sources may be compromised in crowded regions near bright sources (or near bright, extended emission that has not been excluded) where the background has a strong spatial dependence. The emission contributes to the background detection annulus around each source and increases the background variance, hence reducing the significance of the source detection. An example of this problem can be seen near the center of Obsid 6420; see the science study "CSC Release 1: Missing sources in crowded fields" for further details.

• All position angles recorded in the Source Observations Table are defined relative to the tangent plane projection of the individual observation. The 0 degree position angle reference is defined to be parallel to the true North direction at the location of the tangent plane reference point, rather than relative to local true North.

• The source position error ellipse is replaced by a source position error circle in this release. This limitation will be lifted in a future release of the catalog.

• The deconvolved source extent is determined using a circular Gaussian parameterization in this release, rather than a rotated elliptical Gaussian parameterization. This limitation will be lifted in a future release of the catalog.

• Users of the Chandra Interactive Analysis of Observations (CIAO) data analysis package prior to version 4.1 will be unable to use source region files (reg3) if the region either (a) intersects the edge of a chip, or (b) intersects an ACIS read-out streak.

• 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.

• The mean chip coordinates (chipx, chipy) of a source are computed without including the effect of the mean dy, dz, dtheta offsets from the aspect solution. For some observations these offsets are of order 15 arcsec (~30 ACIS pixels) in both dy and dz, so the mean chip coordinates may be uncertain by this amount.

Specific Caveats and Limitations for certain ObsIds

• The catalog has no sensitivity on the ACIS readout streak associated with the very piled-up (cratered) bright source on ObsId 5511.
• ACIS event energy threshold caveat