This page is for the CSC 2.0 release. Please see the CSC home page for the latest release.

Note

Be sure to read the caveats prior to using these data!

Source properties from CSC 2.0 are now being made available to the community as they become available.

Contents

• Data Access
• Column definitions
• Caveats

Data Access

The Chandra X-ray Center is pleased to announce that the November 2017 pre-release for the CSC 2.0 is now available:

• prerelease_srclist.fits.gz which is a FITS table file (51 Mb compressed and 906 Mb uncompressed), whose content is described below;
• and pd2-invalid-detections.txt, which is an ASCII file listing the 16 detections from pd2 that are not included in this pre-release. There are two columns in this file: detect_id, which lists the DETECT_ID value in the pd2 preliminary_detlist.2017-09.fits file; and reason, which is fov if the detection lies outside the region used for the catalog, or qa if the detection was removed by a quality-assurance step after the pd2 release.

This release extends the second preliminary detection list (pd2), released in mid-September 2017, by:

1. identifying multiple detections of the same X-ray source (i.e. which detections from the pd2 table are from the same source),
2. combining the positions of the multiple detections to determine the location of the source, including an error estimate,
3. providing an estimate of the source flux by combining the measurements presented in the pd2 release;
4. and removing a small-number of invalid detections.

The caveats section must be reviewed before using this data.

This version of the pre-release source list is constructed from a full run of Chandra observations that will be included in release 2.0 of the CSC, and contains 315875 unique X-ray sources on the sky. Of these, 77% (243790) are labelled as a TRUE source, with the remaining 72085 sources as MARGINAL, based on the detection likelihood (as stored in the LIKELIHOOD_CLASS column). The list is labelled as a pre-release because a small number (less than 0.1%) of sources in this list may not be included in the final catalog release following the final statistical quality-assurance analysis step.

The sources are formed from 374349 detections from the pd2 preliminary detections release, of which 371427 detections were uniquely mapped to a source, and only 2922 detections (0.8%) with ambiguous matches (that is, the detection is associates with two or more sources).

Column Definitions

unix% dmlist prerelease_srclist.fits blocks

--------------------------------------------------------------------------------
Dataset: prerelease_srclist.fits
--------------------------------------------------------------------------------
 
     Block Name                          Type         Dimensions
--------------------------------------------------------------------------------
Block    1:                                Null        
Block    2: SRCLIST                        Table        28 cols x 315875   rows

SRCLIST HDU: PRE-RELEASE SOURCE LIST

Caveats

Matching Sources to Detections

Each source is matched to one or more detections included in the CSC 2.0 pd2 preliminary detections list. The matching detections can be identified using the DETECT_NUM, DETECT_ID, and MATCH_TYPE columns. The DETECT_NUM column indicates how many detections were combined to form this source, and so indicate the number of entries to use in the DETECT_ID and MATCH_TYPE arrays. The DETECT_ID column consists of an array of the detections that are matched to the source; these are the same names as used in the preliminary detections list (only the pd2 preliminary detections list should be used). Each DETECT_ID has a corresponding entry in the MATCH_TYPE column. A value of "u" means that the detection is a unique match to the source, i.e., there is no other source that could be associated with that detection. Therefore the detection properties should be a direct measure of the source properties. If MATCH_TYPE is instead "a" then the detection is an ambiguous match to the source, i.e, there is more that one source associated with the detection. Ambiguous matches typically happen when an off-axis detection (with a large PSF) is resolved into multiple sources by an overlapping observation where the same region is observed on-axis. In such a case, the off-axis detection will be ambiguously matched to all of the resolved sources. The aperture photometry from the ambiguous detection can be used as an upper limit for the flux of all of the matched sources at the time of the observation, but most other detection properties can not be used to constrain the source properties.

There are 16 detections in the pd2 table which have since been removed from the catalog. They are given in the text file pd2-invalid-detections.txt, which lists the DETECT_ID value along with the reason for its removal: fov indicates that the detection falls outside of the valid region defined for the catalog (i.e. the detection is close to the edge of the detector or falls on an excluded region); or qa, which means that the detection was removed by a quality-assurance check made after the pd2 release.

Positions and position errors

The pre-release source list includes unique X-ray sources on the sky, identified by matching detections from multiple observation stacks that overlap the same region of the sky. The source positions are computed by combining the positions (and associated uncertainties) of detections in the CSC 2.0 pd2 preliminary detections list that are uniquely matched (i.e., have a MATCH_TYPE of "u") to the source. The reported position uncertainties include a component to account for the absolute uncertainty in the astrometry relative to the ICRS reference frame. This uncertainty, 0.29", is derived from comparisons of source positions from release 2.0 detections relative to the Sloan Digital Sky Survey, and is added in quadrature to the position uncertainty combined from the individual detections.

While the true position error should be consistent with the reported position error for ~90% of the sources, the ~10% of sources for which this is not the case are not distributed randomly. There are a small number of observation stacks for which the absolute error may be significantly larger than 0.29" (up to a maximum of ~2.0″).

The position errors are reported in the form of an error ellipse. The ellipse angle in the pre-release source list defines 0° along the +X axis (i.e., the same definition as used by the DS9 imager); however, true astronomical position angles (measured from North through East) will be used in the CSC 2.0 release.

Source names

Source names of the form 2CXO Jhhmmss.s+/-ddmmss are assigned based on the computed source positions. In 15 cases, the positions of two sources resolves to the same source name using this scheme; in such cases, an "A" is appended to the name of one of the sources. These source names will be used in the final catalog release.

Aperture photometry

As a convenience, average per-band fit amplitudes are provided as a proxy for the average source flux. These amplitudes are computed by combining the amplitudes (and associated uncertainties) of detections in the CSC 2.0 pd2 preliminary detections list that have been uniquely matched (i.e., have a MATCH_TYPE of "u") to the source, using the prescription of Barlow (2004; Asymmetric Statistical Errors, arXiv:physics/0406120). While the average amplitudes are expected to be reasonably reliable for relative photometry on a statistical basis, systematic uncertainties for a single source may be large.

Temporal variability of the sources is not considered when computing the average amplitudes. We strongly recommend that users compare the average amplitudes with the amplitudes for the corresponding individual detections listed in the CSC 2.0 pd2 preliminary detections list.

Caveats regarding fit amplitudes in the description of the CSC 2.0 pd2 preliminary detections list also apply to the average per-band fit amplitudes reported here.

Miscellaneous Notes

The sources have undergone all standard quality assurance reviews, and except for the caveats noted herein should match those that will be included in the CSC 2.0 release (the list is labelled as a pre-release because a small number - less than 0.1% - of sources may be dropped from the catalog following the final statistical quality-assurance analysis step).

Sources are categorized as ‘MARGINAL’ or ‘TRUE’ (recorded in the LIKELIHOOD_CLASS column) depending on their likelihood (from the LIKELIHOOD column). These categorizations are those that will appear in the CSC 2.0 release.

Only point and compact detections are included. Extended (“convex hull”) detections are not included in the pre-release source list.