CSC 2.0 pd2 Preliminary Detections List (18 September 2017)
The November 2017 prerelease 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 prerelease page for more information.
Be sure to read the caveats prior to using these data!
Contents
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 preliminarydetections table below;
 stacks.pd2.txt which is an ASCII format file (spaceseparated 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 perstack images available in Table 1, the approximate outline of each stack as a FITSformat 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 highlypiled 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 ultrasoft ('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 spaceseparated 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.
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Figure 1: The detections in a stack covering M31
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
Column name  Units  Comment  Data type  Notes  Example 

DETECT_ID  Stack_id.component  string (29 characters)  'acisfJ0123456p012345_001.0001'  
LIKELIHOOD  Highest source log likelihood  double  Highest source likelihood  100.0  
SRC_QUALITY  Preliminary source quality  string (8 characters)  Values are 'TRUE' or 'MARGINAL' and are based on comparison of likelihood with draft catalog thresholds.  'TRUE'  
EBAND  Energy band of maximum likelihood value  string (1 character)  Energy band corresponding to likelihood value; Science energy band designation (i.e., lower case) not detect energy band: one of 'b', 'u', 's', 'm', 'h', or 'w'.  'b'  
THETA  arcmin  Highest likelihood src model mean offaxis ang  double  Corresponds to fitted [x,y]  0.0 
STREAK_SRC_FLAG  Is source on a streak?  logical  'F'  
RA  deg  Highest likelihood source model position RA  double  20.94000  
DEC  deg  Highest likelihood source model position Dec  double  1.39583  
ERR_ELLIPSE_RA  deg  95% confidence error ellipse position RA  double  95% confidence error ellipse RA in deg; 0.0 <= ERR_ELLIPSE_RA < 360.0  20.94000 
ERR_ELLIPSE_DEC  deg  95% confidence error ellipse position Dec  double  95% confidence error ellipse Dec in deg; 90.0 <= SRC_ERR_ELLIPSE_DEC_B <= +90.0  1.39583 
ERR_ELLIPSE_R0  arcsec  95% confidence error ellipse semimajor axis  double  95% confidence error ellipse semimajor axis radius in arcsec  0.35 
ERR_ELLIPSE_R1  arcsec  95% confidence error ellipse semiminor axis  double  95% confidence error ellipse semiminor axis radius in arcsec  0.30 
ERR_ELLIPSE_ANG  deg  95% confidence error ellipse smaj axis rotang  double  95% confidence error ellipse rotation angle of semimajor axis; 0.0 <= ERR_ELLIPSE_ANG < 180.0; 0.0 along SKY +X, 90.0 along SKY +Y  60.0 
SRC_RDATA_B  count  Raw data counts in ECF90 aperture ACIS b band  long  100  
SRC_AMPL_B  photon/cm**2/s  Best fit point src model amplitude ACIS b band  double  1.00E08  
SRC_AMPL_LOLIM_B  photon/cm**2/s  68% confidence lower limit on SRC_AMPL_B  double  9.00E09  
SRC_AMPL_HILIM_B  photon/cm**2/s  68% confidence upper limit on SRC_AMPL_B  double  1.10E08  
SRC_RDATA_S  count  Raw data counts in ECF90 aperture ACIS s band  long  30  
SRC_AMPL_S  photon/cm**2/s  Best fit point src model amplitude ACIS s band  double  3.00E09  
SRC_AMPL_LOLIM_S  photon/cm**2/s  68% confidence lower limit on SRC_AMPL_S  double  2.00E09  
SRC_AMPL_HILIM_S  photon/cm**2/s  68% confidence upper limit on SRC_AMPL_S  double  4.00E09  
SRC_RDATA_M  count  Raw data counts in ECF90 aperture ACIS m band  long  50  
SRC_AMPL_M  photon/cm**2/s  Best fit point src model amplitude ACIS m band  double  5.00E09  
SRC_AMPL_LOLIM_M  photon/cm**2/s  68% confidence lower limit on SRC_AMPL_M  double  4.00E10  
SRC_AMPL_HILIM_M  photon/cm**2/s  68% confidence upper limit on SRC_AMPL_M  double  6.00E09  
SRC_RDATA_H  count  Raw data counts in ECF90 aperture ACIS h band  long  20  
SRC_AMPL_H  photon/cm**2/s  Best fit point src model amplitude ACIS h band  double  2.00E09  
SRC_AMPL_LOLIM_H  photon/cm**2/s  68% confidence lower limit on SRC_AMPL_H  double  1.00E09  
SRC_AMPL_HILIM_H  photon/cm**2/s  68% confidence upper limit on SRC_AMPL_H  double  3.00E09  
SRC_RDATA_W  count  Raw data counts in ECF90 aperture HRC w band  long  TNULL  
SRC_AMPL_W  photon/cm**2/s  Best fit point src model amplitude HRC w band  double  NaN  
SRC_AMPL_LOLIM_W  photon/cm**2/s  68% confidence lower limit on SRC_AMPL_W  double  NaN  
SRC_AMPL_HILIM_W  photon/cm**2/s  68% confidence upper limit on SRC_AMPL_W  double  NaN  
EBAND_EXT  Energy band of extended source model properties  string (1 character)  Energy band corresponding to reported extended source parameters: set to ' ' if not evaluated, otherwise one of 'b', 'u', 's', 'm', 'h', or 'w'.  's'  
EXTSRC_CLASS  Extended source classification  string (8 characters)  'POINT', 'EXTENDED', or 'POSSIBLE'  'POSSIBLE'  
EXT_RDATA  count  Raw data counts in source region aperture  long  40  
EXT_AMPL  photon/cm**2/s  Best fit ext src model amplitude  double  2.50E09  
EXT_AMPL_LOLIM  photon/cm**2/s  68% confidence lower limit on EXT_AMPL  double  1.50E09  
EXT_AMPL_HILIM  photon/cm**2/s  68% confidence upper limit on EXT_AMPL  double  3.50E09  
EXT_SMAJ  arcsec  Best fit extended source model semimajor axis  double  5.0  
EXT_SMIN  arcsec  Best fit extended source model semiminor axis  double  3.5  
EXT_ROTANG  deg  Best fit extended source model rotation angle  double  30.0  
EXT_SMAJ_LOLIM  arcsec  68% confidence lower limit on EXT_SMAJ  double  4.5  
EXT_SMAJ_HILIM  arcsec  68% confidence upper limit on EXT_SMAJ  double  5.5  
EXT_SMIN_LOLIM  arcsec  68% confidence lower limit on EXT_SMIN  double  3.0  
EXT_SMIN_HILIM  arcsec  68% confidence upper limit on EXT_SMIN  double  4.0  
EXT_ROTANG_LOLIM  deg  68% confidence lower limit on EXT_ROTANG  double  10.0  
EXT_ROTANG_HILIM  deg  68% confidence upper limit on EXT_ROTANG  double  50.0 
Caveats

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.

The detection list is labelled as preliminary because:

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 qualityassurance analysis step;

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 qualityassurance review.


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.

Offaxis detections from one observation stack may correspond to multiple detections in another observation stack where the detections are onaxis. 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.


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 piledup (cratered) sources in the pd1 release has been addressed.

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.


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 Xray 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.


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 backilluminated and frontilluminated ACISS 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 higherthannominal false detection rate either near these maskedout 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
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.