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

Note

The November 2017 pre-release augments this release by combining overlapping detections to form sources. The pre1 release table includes the source names, position and errors, an estimate of an average flux, and the detections used to form each source. It also removes 16 detections from the pd2 release. Please consult the pre-release page for more information.

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

Contents

• Data Access
• Column definitions
• Caveats
• Figures and Tables

Data Access

The September 2017 preliminary detections list is available at:

• preliminary_detlist.fits.gz (55 Mb compressed and 129 Mb uncompressed) which is described in the preliminary-detections table below;
• stacks.pd2.txt which is an ASCII format file (space-separated columns) that extends the data in Table 1 to include the individual Chandra observations included in a stack (in the format of obsid_obi) and the nominal location for each stack;
• the tar files coverage.pd2.png.tar.gz, coverage.pd2.fits.tar.gz, and coverage.pd2.reg.tar.gz which contain the per-stack images available in Table 1, the approximate outline of each stack as a FITS-format table, and the outlines as DS9 ASCII region files, respectively (as discussed in the Caveats section below, the actual survey area is more restrictive than the data stored in these files);
• and detections_only_in_pd1.txt lists those detections from the pd1 release that have been removed in the pd2 release, as described in the Caveats section below.

This release adds the missing fields from the May 2017 pd1 release, and so contains all Chandra observations that will be included in release 2.0 of the Chandra Source Catalog. Compared to the pd1 release, the number of stacks has increased to 7287 - it was 7247 - and the number of detections has increased by 3 percent. As well as adding in more fields, the estimate of the systematic error for the absolute astrometry has been increased from 0.19 to 0.29 arcsec, and problem affectings a small number of detections (those that are highly-piled up or were missing postional errors) have been addressed.

It contains the results of Maximum Likelihood Estimator (MLE) fits to the candidate source detections. The column definitions and Caveats for this file can be found below. The file contains 374365 detections, with 289592 labelled as SRC_QUALITY = ‘TRUE’ and 84773 (23%) with SRC_QUALITY = ‘MARGINAL’. When split by EXTSRC_CLASS, there are 369654 detections with a value of ‘POINT’, 3849 (1.0%) with ‘EXTENDED’, and 862 (0.2%) with ‘POSSIBLE’. The breakdown by EBAND—the energy band correponding to the measured values—there are 278296 broad band ('b'), 475 ultra-soft ('u'), 27931 soft ('s'), 27209 medium ('m'), 35195 hard ('h'), and 5259 wide (HRC) band ('w') rows; the band definitions are the same as in release 1. There are 368 detections with STREAK_SRC_FLAG = True.

The 7287 stacks used in this release are listed in Table 1. This data is available as a text file - with space-separated columns - at stacks.pd2.txt. Note that of the 7287 stacks, there are 37 which have no detections. Figure 1 shows an example stack from the preliminary detection list; this data is provided for all stacks from the links in the first column of Table 1, or from the tar files mentioned above.

Figure 1: The detections in a stack covering M31

[Version: full-size]

Figure 1: The detections in a stack covering M31

The image shows the detections in the stack acisfJ0042424p411621_001, which covers part of M31, and is constructed from 33 separate Chandra observations. The solid circles mark the 319 TRUE detections from this stack and the open squares the 38 MARGINAL detections. The grey background indicates the approximate sky coverage of the stack (it slightly over-estimates the coverage since it is based on the Chandra Field Of View files whereas the catalog uses a more-restrictive mask).

Column Definitions

unix% dmlist preliminary_detlist.fits blocks
 
--------------------------------------------------------------------------------
Dataset: preliminary_detlist.fits
--------------------------------------------------------------------------------
 
     Block Name                          Type         Dimensions
--------------------------------------------------------------------------------
Block    1:                                Null        
Block    2: DETLIST                        Table        48 cols x 374365   rows

DETLIST HDU: PRELIMINARY DETECTIONS LIST

Caveats

1. This version of the preliminary detections table is constructed from a full run of Chandra observations that will be included in release 2.0 of the CSC, and contains 374365 detections from 7287 observation stacks. Further details of the stacks are given in Table 1 and in the stacks.pd2.txt ASCII file.

   There are 188 detections which were included in the pd1 release but have since been rejected while assessing the quality of the catalog, and so are not included in this release. The file detections_only_in_pd1.txt contains the list of the 188 DETECT_ID values for these detections.

2. The detection list is labelled as preliminary because:

   1. a small number (< 0.1%) of detections that are included in this version of the preliminary detections list may not be included in the final catalog release following the final statistical quality-assurance analysis step;

   2. a small number of detections located close to the Galactic Center that are included in this version of the preliminary detections list may not be included in the final catalog as a result of manual quality-assurance review.

3. The preliminary detections table includes detections and not sources. In particular:

   • Sources that are detected in multiple observation stacks are not combined in any way. Each detection is reported separately.

   • Off-axis detections from one observation stack may correspond to multiple detections in another observation stack where the detections are on-axis. These matches will be resolved in the second phase of the catalog processing.

   • As a consequence, source names have not yet been assigned. Instead, an identifier has been provided for each detection. Note that the identifiers included in this version of the preliminary detections table may not match the identifiers included in the August 2015 (pd0) version of the table because a significant fraction of the observation stacks required complete reprocessing since then.

4. Detections:

   A small number of compact detections identified as definitely or possibly extended (EXTSRC_CLASS = ‘EXTENDED’ or ‘POSSIBLE’) may consist of a undetected close association (typically a pair or triple) of point detections. These will not be addressed in the CSC 2.0 release.

   The problem with extremely piled-up (cratered) sources in the pd1 release has been addressed.

5. Positions and position errors:

   • Position errors are typically reported in the form of an error ellipse that records the statistical confidence limits derived from a Markov chain Monte Carlo (MCMC) analysis of the fitted source model. The ellipse angle in the preliminary detections table 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.

   • Reported position errors do not include the absolute uncertainty in the astrometry relative to the reference frame. The radial error derived from comparisons of source positions from release 2.0 relative to the Sloan Digital Sky Survey is 0.29″, and this number should be added in quadrature to the reported position errors to get the best estimate of the absolute position uncertainty on the sky.

     Note that this is an increase from the systematic error derived from release 1 of the catalog (0.16″ radial), which was the suggested value to use with the pd1 release.

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

   The small number of detections in the pd1 release with missing positional errors has been fixed in this release.

6. Aperture photometry:

   • Fit amplitudes are provided as a proxy for the detection flux: while these are expected to be reasonably reliable for relative photometry on a statistical basis, systematic uncertainties for a single detection basis may be large. As a guide to possible biases, Figure 2 compares - for a subset of the pd2 release - the fit amplitudes to aperture photometry calculated using the Bayesian X-ray photometry scheme that will be used in the CSC 2.0 release.

   • Reported fit amplitudes are incorrect (significantly too large) for one or more energy bands for ~3,500 detections. The broad band flux is equal to the sum of the soft, medium, and hard band fluxes. These cases can be identified by comparing the sum of the reported soft, medium, and hard band fit amplitudes with the reported broad band fit amplitude.

7. Miscellaneous notes:

   • The detections 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.

   • Detections are categorized as ‘MARGINAL’ or ‘TRUE’ (recorded in the SRC_QUALITY column) depending on their likelihood. 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 preliminary detections table, but will appear in the CSC 2.0 release.

   • Detections near the edges of the field of each observation, or near the gap between adjacent back-illuminated and front-illuminated ACIS-S CCDs, or on bright ACIS readout streaks, are masked out. This is intentional because these detector areas had an excessively high false source rate in release 1 of the catalog. In the full CSC 2.0 release, the detection sensitivity will be set to zero in these areas. However, detection sensitivity information will not be available until completion of the second major catalog processing phase. There may be a higher-than-nominal false detection rate either near these masked-out regions or close to areas of extended emission. If this is a concern, we recommend that users review images of the individual observations from the Chandra Data Archive. The stacks.pd2.txt file contains a breakdown of the observations used to create each stack.

     The images provided via Table 1 that show the coverage of each field - such as the M31 stack shown in Figure 1 - are based on the Field Of View files of the observations that form each stack, and do not include the masking discussed above.

Figures and Tables

Figure 2: Comparison of source photometry

[Version: full-size, PDF]

Figure 2: Comparison of source photometry

Correlation between the fitted source amplitude and detailed Bayesian X-ray aperture photometry analysis for 14,360 detections, in the ACIS broad energy band. Note that the correlation is somewhat worse for faint detections categorized as ‘MARGINAL’. The correlation has not been demonstrated for energy bands other than the ACIS broad band; however, we expect that the correlations in the other bands should be similar.

The table listing the stack names, along with access to the stack outline file, and other ancillary data, is available on the stack table page.