Chandra Source Catalog Release 2.0
While all sources have undergone standard quality assurance evaluations, some may be deleted or have their properties revised as a result of further quality-assurance analyses step prior to the official CSC 2.0 release.
The processing status page is regularly updated to indicate how many sources have been processed and added to the database.
The Chandra Source Catalog (CSC) is ultimately intended to be the definitive catalog of X-ray sources detected by the Chandra X-ray Observatory. To achieve that goal, the catalog will be released to the user community in a series of increments with increasing capability. The first official release of the CSC 2.0 will include 10,382 Chandra ACIS and HRC-I imaging observations released publicly through the end of 2014. When complete, CSC 2.0 will provide information for about 370,000 unique sources in several energy bands:
- the full detection list from stacks of fields with aim points within 60″
- unique source determination and naming by merging detections across multiple overlapping stacks
- full extraction of source properties (aperture photometry, variability analysis, and spectral properties)
The CSC version 2.0 contains positions and multi-band count rates for the sources, as well as derived spatial, spectral, and temporal calibrated source properties that may be compared with data obtained by other telescopes. The CSC also includes associated data products for each source, including images, photon event lists, light curves, and spectra. It improves upon the previous release, CSC 1.1 by increasing the number of Chandra observations used, using a new detection method, the inclusion of fainter X-ray sources, and additions to the set of source properties (described in more-detail below).
Each distinct source on the sky (i.e., object at a specific RA and Dec) is recorded in a single "master sources" table entry, one or more stacked observation detections table entries, and one or more "per-observation detections" table entries. Sources are initially detected at the stack level from combined data of coaligned pointings; properties determined at the stack level are stored in the stacked observation detections table, while properties in each individual observation are in the per-observation detections table entries. The master sources entry is the best estimate of all the properties of a source, based on the data extracted from the individual source entries. The Catalog Organization page contains further details.
Release 2.0 Overview
The following list highlights the contents of this release, and represents major improvements over release 1.1 of the CSC.
All public imaging Chandra imaging data (ACIS and HRC-I) through the end of 2014 (release 1.1 includes data up to the end of 2009). There are 9,573 ACIS observations and 809 HRC-I observations included in release 2.0.
All observations of the same area of sky that are centered within one arcminute are stacked together, increasing the depth of the catalog. Detections from different stacks are cross-matched to derive the unique CSC 2.0 source list.
Source detection can reach as deep as five counts for on-axis sources.
PSF modeling is used to determine source location and properties, improving off-axis sensitivity.
The background maps have been improved, enhancing reliability of source detection in fields with spatially-varying diffuse emission.
Bright extended sources are included.
Edge effects are handled better.
Evaluation of statistical errors on source properties is improved.
Multi-band aperture photometry, time variability, and hardness ratios are determined by means of Bayesian methods.
Expanded list of models used for spectral fits of bright sources.
As in CSC 1.1, tabulated properties and associated products, in FITS format, will be available for further analysis by the community.
Users are urged to review the catalog Caveats and Limitations prior to using the CSC for their scientific investigations. Questions about the CSC may be submitted to the CXC Helpdesk; refer also to the FAQs section of the website for answers to frequently asked questions.
The Chandra Source Catalog
Release 2.0 of the Chandra Source Catalog (CSC 2.0) includes data from observations released publicly through the end of 2014. When complete, CSC release 2.0 will include information for about 370,000 source detections from 10,382 Chandra ACIS and HRC-I imaging observations.
Compared with release 1.1 of the catalog, CSC 2.0 represents a major improvement in both quantity of data included (CSC1.1 included data up to 2009) and approach to data processing, resulting in fainter source thresholds and better defined source properties.
- Field Stacking
Exploiting the unique resolution and very low background of Chandra data, we were able to improve significantly the depth of the catalog, by stacking (co-adding) multiple observations of the same field prior to source detection. This is a significant departure from CSC1.1 where sources were detected in each field separately and their properties averaged (a method also followed in the XMM catalogs, where the high background does not justify stacking). To minimize the impact of the variation of the Chandra point spread function (PSF) across the field, source detection in CSC 2.0 is constrained to run on stacks of observations that have pointings co-located within 60″, and that were obtained using the same instrument (ACIS or HRC-I).
- Improved Two-step Source Detection
A new method is used that can reliably detect point sources down to roughly 5 net counts on-axis for exposures shorter than the median Chandra observation duration. Improved background maps (see below) allow point source detection in areas with diffuse emission and near the edge of fields. As for CSC1.1, source detection is performed first using the CIAO wavelet source detection tool (wavdetect), but in CSC 2.0 the tool parameters are updated to identify fainter candidate detections. The resulting large false detection rate is filtered by a second step in processing (new to CSC 2.0). A new maximum likelihood estimator (mle) uses the Sherpa modeling and fitting package to fit the PSF model to each candidate detection, to evaluate the likelihood that the detection is real. Candidate detections are classified as TRUE or MARGINAL in the catalog, depending on their likelihood.
- PSF Modeling
Since the Chandra PSF is highly position dependent, the local PSF was calculated for each source position prior to fitting. Fitting with the local PSF also improves source astrometry, particularly for larger off-axis angles where PSF asymmetries can bias wavdetect position determinations.
- Improved Background Maps
CSC 2.0 uses a new Voronoi-tessellation background tool (mkvtbkg) to create improved background maps prior to source detection. These background maps perform better than those used in earlier releases in regions where the background intensity is changing rapidly (e.g., in areas of extended emission near the centers of galaxies) and at large off-axis angles.
- Inclusion of Bright Extended Sources
mkvtbkg identifies candidate compact (point-like or nearly so) detections as well as regions of extended emission. The latter capability allows bright, extended sources to be included in the CSC for the first time. Such sources are identified with a bounding convex hull polygon in release 2.0 of the catalog. Sets of polygons with multiple intensity thresholds will be available (as FITS format data products) for end users who wish to perform more detailed analysis of detected extended sources.
- Better Handling of Edge Effects
Unlike earlier releases, sources detected near the edges of the (stacked) field of view, in the gaps between ACIS back-illuminated and front-illuminated CCDs (on the ACIS-S array), and on readout streaks associated with saturated, bright sources, are excluded from CSC 2.0. In CSC1.1, a significant fraction of sources in such regions were determined to be false. CSC 2.0 includes multi-band limiting sensitivity maps computed on a fine-grained (~4″×4″) scale so that users can identify regions that are included in/excluded from the catalog.
- Cross-matching of Stack Detections
This step, using the same algorithm as in CSC1.1, compares positions and positional errors for detections from multiple overlapping stacks, to determine the CSC 2.0 unique source list.
- Improved Evaluation of Source Properties
Like earlier releases, CSC 2.0 includes numerous raw measurements for each detected source, as well as scientifically useful properties (and associated errors) derived from the observations in which a source is located. Positions include error ellipses for brighter sources and circular errors for faint sources. Multi-band aperture photometry will be determined using a Bayesian approach to compute photometric probability density functions that will subsequently be used directly for computing such quantities as cross-band hardness ratios and temporal variability measures. This avoids some inconsistencies present in CSC1.1 where these properties were computed independently.
- Expanded Model List for Spectral Fits
Spectral fits computed using multiple models (e.g., absorbed power-law, black body, and bremsstrahlung) will be provided for brighter sources. The minimum count matches that of CSC 1.1 (150 counts), but the analysis now includes simultaneous fits to multiple detections of the same source.
- Extra Data Products
In addition to the tabulated properties, CSC 2.0 will provide FITS format data products that include full field event lists, multi-band images, exposure maps, limiting sensitivity maps, merged source lists, and extended source polygons. Source region data products include per-source-region event lists, multi-band images, photometry probability density functions, exposure maps, pulse-invariant spectra, spectral response matrices, and optimally-binned light curves.
To facilitate user access to the catalog as quickly as possible, production has been split into four phases:
Phase 1 (completed)
The first phase re-calibrates all 10,382 Chandra data sets that are included in release 2.0, generates backgrounds, performs source detection, and then evaluates the candidate detections' likelihoods using the mle tool. A subset of the information contained in the resulting merged source lists is combined into a preliminary detections list that is periodically updated and made available through this website. The detections list includes positions, likelihoods, extents, and associated errors. A fitted intensity that is a reasonably good proxy for aperture photometery (except in the Poisson regime) is included. Some error estimates are preliminary.
This phase has been completed and made available as the pd2 release. This release contains information on 374,365 detections.
Phase 2 (completed)
In the second phase, overlapping detections are combined—using the same master-match algorithm as used in release 1.1—to create sources. Basic positional properties, including an error estimate, are calculated for these sources, along with an average flux estimate based on the values included in the pd2 release.
This phase has been completed and made available as the pre1 pre-release. This table contains information on 315,875 sources, created from 374,349 detections (16 detections have been removed from the pd2 list due to quality-assurance checks).
Phase 3 (ongoing)
The third phase is when the full set of source properties are calculated; this includes the aperture photometry for each source, together with derived properties, such as: hardness ratios, spectral information, and variability measures. In addition to the tabulated properties, CSC 2.0 will provide FITS format data products that include full-field event lists, multi-band images, exposure maps, and spectral files: source and background PHA files, along with the necessary response information (ARF and RMF) to allow spectral analysis with systems such as Sherpa, the Interactive Spectral Interpretation System, or XSPEC.
This phase started in October 2017, and the results will be made be made available to the community via the CSCview application. The first set of sources will be made available after a set of quality-assurance checks have been completed. This is expected to be finished by the end of November 2017. After this, new sources will be added as they are processed. Since the catalog has not been completed, and so final checks have not been made, the data at this stage should still be considered preliminary (sources may be deleted or have their properties changed). The World Wide Telescope visualization provides a quick way to view the progress of this phase, and more details are available on the processing status page.
The full release of CSC2.0 data, which includes a final statistical quality-assurance analysis step, is expected in February 2018. The data will be augmented with limiting-sensitivity maps, and the release of the catalog of bright, extended X-ray sources ("convex hull" sources).