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Last modified: 4 December 2017

URL: http://cxc.harvard.edu/csc/caveats.html

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 inquiry.

  1. Catalog Data:
    1. General
    2. Specific Caveats and Limitations applicable through CSC release 2
    3. Specific Caveats and Limitations for certain ObsIds
    4. Flux Correcting Very Bright Sources[New]
  2. Catalog Interfaces
    1. CSCview
    2. ADQL
    3. The contributed scripts

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.

Users of catalog current 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.

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

  • 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.
  • Some data products such as PHA files (pha3) contain 'pixel masks', a new feature. Users of the Chandra Interactive Analysis of Observations (CIAO) data analysis package prior to version 4.10 (release scheduled for mid 2018) may encounter problems using these files with some tools.
  • 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.
  • 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).

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.

Flux Correcting Very Bright Sources[New]

For some very bright sources, such as 2CXO J150842.1+332802, the method used to correct the energy flux in the source aperture for background contributions, using energy flux in the background aperture, may fail, resulting in NaN's for flux_aper or flux_aper90. Other flux quantities, such as photflux_aper, or src_rate_aper, are unaffected.


Catalog Interfaces

CSCview

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

Interoperability with other tools

  • The CSCview Send feature has been found to be incompatible with the Catalog Tool in DS9 version 6.0, but should work with versions 6.1 and onward.
  • 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.

ADQL

Metadata returned from a query

The metadata returned from an ADQL query is incomplete, as it is missing the units and any UCD terms for most columns.

The contributed scripts

The search_csc and obsid_search_csc scripts currently do not support the CSC 2 database, and so will only return values from the CSC 1.1 release.


Last modified: 4 December 2017
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