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

This version has been superseded by the September 2017 release

Please see the September 2017 preliminary detections list (CSC 2.0 pd2).

Contents

• Data Access
• Column definitions
• Caveats

Data Access

The March 2017 preliminary detections list is available at:

• preliminary_detlist.fits.gz (53.4 Mb compressed and 124.4 Mb uncompressed)

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 362182 detections, with 279549 labelled as SRC_QUALITY = ‘TRUE’ and 82633 (23%) with SRC_QUALITY = ‘MARGINAL’. When split by EXTSRC_CLASS, there are 358382 detections with a value of ‘POINT’, 3183 (0.9%) with ‘EXTENDED’, and 617 (0.2%) with ‘POSSIBLE’. The breakdown by EBAND—the energy band correponding to the measured values—there are 271687 broad band ('b'), 395 ultra-soft ('u'), 26995 soft ('s'), 26524 medium ('m'), 31297 hard ('h'), and 5284 wide (HRC) band ('w') rows; the band definitions are the same as in release 1. There are 354 detections with STREAK_SRC_FLAG = True.

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 362182   rows

DETLIST HDU: PRELIMINARY DETECTIONS LIST

Caveats

1. This version of the preliminary detections table is constructed from a partial run of Chandra observations that will be included in release 2.0 of the CSC, and includes 362,182 detections from 7,247 observation stacks.

2. Roughly 0.5% of observation stacks have not yet completed processing or are awaiting manual quality assurance review. The list of observation stacks not included in this version of the preliminary detections table is reported in Table 1.

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 second major catalog processing phase.

   • 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 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 close association (typically a pair or triple) of point detections. These will not be corrected in the CSC 2.0 release.

   • Detections that are extremely piled-up (cratered) have likelihood = +Inf. A very small number (of order 10) of these detections are incorrectly classified and will be corrected in the CSC 2.0 release.

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.

   • If the MCMC analysis did not converge (typically due to low S/N) then a circularly symmetric estimate of the position errors is provided. For a small number (~240) detections, this estimate is missing and the reported errors are blank. This will be corrected in the CSC 2.0 release of the catalog.

   • Reported position errors do not include the absolute uncertainty in the astrometry relative to the reference frame. The systematic error derived from release 1 of the catalog (0.16″ radial) should be a reasonable estimate of this value, 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.

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 1 compares - for a subset of the pd1 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 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.

Comparison of source photometry

[Version: full-size, PDF]

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.