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Last modified: 15 November 2017


Chandra Source Catalog Release 2.0 (CSC 2.0)

[The detections are distributed across the sky, with various features, such as the galactic plane and Magellanic clouds, visible]

[Version: full-size]

The CSC 2.0 pre1 release showing the number of detections per stack (circle size) and the number of observations per stack (circle color), in Galactic coordinates.

CSC 2.0 includes data from 10,382 Chandra ACIS and HRC-I imaging observations released publicly through the end of 2014. When complete, CSC2.0 will provide information for about 370,000 unique detections 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, and ● full extraction of source properties (aperture photometry, variability analysis, and spectral properties). The CSC 2.0 catalog - and data products - will be accessible via our standard catalog interface, CSCview.

Data Access Table
Designation Date Description and Notes
CSC 2.0 pre1 2017-11-15

This pre-release adds source designations - that is, location and error; an estimate of the average aperture photometry; and the detections that form each source - to the CSC 2.0 pd2 release. The list contains 315,875 sources, formed from 374,349 detections (16 detections from the pd2 release have been rejected).

Note that this pre-release does not include the full characterisation for each source, such as the full set of aperture photometry measurements or spectral properties, nor does it include information on large, extended X-ray sources (the "convex hull" detections).

CSC 2.0 pd2 2017-09-18

Final detections from all 7,287 of the observation stacks in CSC 2.0. The release contains 374,365 positions, likelihoods, extents, a fitted intensity that is a reasonably good proxy for aperture photometry, along with associated errors for these quantities. The CSC 2.0 pd2 data access page provides access to the table as well as a full list of the properties and caveats.

This release extends the pd1 release to all observations that are included in CSC release 2.0, as well as fixing several issues with the pd1 release, as described in the caveats section. It also includes information on the observations used to create each stack, and images showing the approximate sky coverage of each stack.

CSC 2.0 pd1 2017-03-21

This release has been superseded by the CSC 2.0 pd2 release.

CSC 2.0 pd0 2015-08-03

This release has been superseded by the CSC 2.0 pd2 release.

Release 2.0 Overview

The following list highlights the contents of this release, and some of the major improvements over release 1.1 of the CSC (these points are discussed in more detail below):

  • 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 CSC2.0 source list.

    The release 2.0 catalog is constructed from 7,287 stacks, which is comprised of 6,973 ACIS stacks and 314 HRC-I stacks.

  • Source detection reaches 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 for source detection in fields with spatially-varying diffuse emission.

  • The inclusion of bright extended sources.

  • Better handling of edge effects.

  • Improved evaluation of statistical errors on source properties.

  • Bayesian determination of multi-band aperture photometry, time variability, and hardness ratios.

  • Expanded model list 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.

The Chandra Source Catalog

Release 2.0 of the Chandra Source Catalog (CSC2.0) 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, CSC2.0 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 CSC2.0 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).

The catalog is constructed from 10,382 Chandra observations (9,573 ACIS and 809 HRC-I) which, after combining, form 7,287 stacks (6,973 ACIS and 314 HRC).

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 CSC2.0 the tool parameters are updated to identify fainter candidate detections. This results in an unacceptably large false detection rate, requiring a second step in processing (new to CSC2.0). 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

CSC2.0 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 CSC2.0. In CSC1.1, a significant fraction of sources in such regions were determined to be false. CSC2.0 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 CSC2.0 unique source list.

Improved Evaluation of Source Properties

Like earlier releases, CSC2.0 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.

Release Phases

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 on a rolling-release schedule. 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.

Phase 4

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).

Last modified: 15 November 2017
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