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The Chandra Source Catalog will be the definitive catalog of all X-ray sources detected by the Chandra X-ray Observatory. It will provide simple access to Chandra data for individual sources or sets of sources matching user-specified search criteria. The catalog is designed to satisfy the needs of a broad-based group of scientists, including those who may be less familiar with astronomical data analysis in the X-ray regime. For each X-ray source, the catalog will list the source position and a detailed set of source properties, including commonly used quantities such as multi-band aperture fluxes, hardness ratios, temporal variability information, and source extent estimates. In addition to these traditional elements, the catalog will include file-based data products that can be manipulated interactively by the user, including images, photon event lists, light curves, and spectra for each source individually from each observation in which a source is detected. The catalog will be released to the user community in a series of increments with increasing capability. Catalog releases will be carefully controlled, and each release of the catalog will be accompanied by a detailed characterization of the statistical properties of the catalog to a well defined, high level of reliability. Key properties that will be characterized include limiting sensitivity, completeness, false source rates, astrometric and photometric accuracy, and variability information. The first release of the catalog is expected in the fall of 2008. This release will include information about sources detected in public ACIS imaging observations from roughly the first eight years of the Chandra mission. It is not yet clear whether public HRC imaging observations will be included in the first release of the catalog. Only point sources, and compact sources with extents < ∼30 arcsec, will be included. Highly extended sources, and sources located in selected fields containing bright, highly extended sources, will be excluded. The catalog will include detected sources whose flux estimates are at least 3 times their estimated 1 σ uncertainties, typically corresponding to about 10 net source counts on-axis and roughly 20–30 net source counts off-axis. In the first release, multiple observations of the same field will be linked together with a single source name (see the section on catalog organization below), but will not be combined prior to source detection. Therefore the flux cutoff applies to each observation separately. For each detected source and observation, we anticipate that the catalog will include the following tabulated properties (for all sources except where otherwise noted):
In addition, a number of file-based data products will be produced for each source individually in formats suitable for further analysis in CIAO. These include:
For a detailed description of the planned catalog contents, users are presently referred to parts I and III of the Chandra Source Catalog Requirements document. An estimate of the eventual size of the catalog can be obtained by projecting forwards from the sky coverage to date. Observations obtained during the first 6 years of the Chandra mission covered about 160 square degrees on the sky (including ∼80 square degrees down to a flux level of 1.0×10-14 ergs cm-2 s-1), with an estimated 150,000 detectable sources containing at least 10 counts. These numbers will continue to grow as the mission continues, with a 15 year prediction of ∼400,000 sources distributed over ∼400 square degrees, or ∼1% of the sky. User access to the catalog will be through a web-based browser interface in the first instance, with future support for a scripting language interface and virtual observatory workflows. Catalog users will be able to query the tabulated properties, and retrieve the file-based data products data objects for further analysis. Since Chandra is an on-going mission, new observations are released publicly on a continuing basis. To support access to stable released catalog versions, as well as source information extracted from recently released observations, distinct “catalog release views” and “database views” are provided. Catalog release views will provide access to released versions of the catalog. Releases are carefully controlled, infrequent (of order 1 per year), and will be accompanied by a detailed statistical characterization. Once data are included in a catalog release view, then they are frozen in that view, even if they are superseded or deleted in subsequent catalog release or database views. Database views provide a direct access to the catalog database, including new content that may not be present in an existing catalog release. The catalog database will be updated continually as observations are released publicly, and sources are processed. Tabulated data and file-based data products in a database view may be superseded at any time, and the statistical properties of data are not guaranteed. The user will, however, be able select a database view for an arbitrary user-chosen date and time, with the default being the current date and time. We anticipate that users who require a stable, well-characterized dataset will primarily access the catalog through a catalog release view, whereas users who are interested in searching the latest data to identify sources with specific signatures for further study will use a database view. Each identified distinct X-ray source on the sky will be represented in the catalog by a single “master” catalog entry and one or more “source-by-observation” entries (one for each observation in which the source was detected). The master catalog entry records the best-estimates of the tabulated properties for a source, based on the data extracted from the set of observations in which the source was detected. The source-by-observation entries record all of the tabulated properties about a detection extracted from a single observation, as well as the file-based data products, which are observation-specific. All of the tabulated properties included in both the master catalog and the source-by-observation catalog entries will be available for searching via the user interface. Bidirectional links within the database maintain the connections between the master catalog entries and associated source-by-observation entries, so that users can access all observations of a single source seamlessly. During catalog processing, source detections in each observation are matched with source detections in observations that overlap the same region of the sky. Detections that can be matched uniquely are merged to construct the master catalog entry for that source. Because the Chandra point spread function is a strong function of off-axis angle, detections far off-axis in one observation may overlap multiple resolved sources detected on-axis in another observation. Such detections are flagged as confused. The source-by-observation entries are linked to the master catalog entries for each overlapped source (and so are accessible to the user), but the data are not used to compute the master source properties. The user will nevertheless be able to identify all of the X-ray sources in the catalog that could be associated with a detection from a single observation, and vice-versa. |
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The Chandra X-Ray
Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory. 60 Garden Street, Cambridge, MA 02138 USA. Email: cxcweb@head.cfa.harvard.edu Smithsonian Institution, Copyright © 1998-2004. All rights reserved. |
Last modified: 01/24/08