---

What is New in CSC 2.1

Compared to CSC 2.0, CSC 2.1 increases the number of sources in the catalog by about one-third, augmenting the total number of observations included from 10,382 to 15,533 and the number of observation stacks from 7,287 to 10,034. A significant fraction of the new observations added represent new sky coverage compared to CSC 2.0, with the sky coverage increasing from ~559 square degrees in CSC 2.0 to about 730 square degrees in CSC 2.1. Figure 1 shows the sky coverage of release 2.1 of the CSC. The ordinate value is the total sky area included in the CSC that is sensitive to point sources with fluxes in any energy band at least as large as the corresponding value on the abscissa.

CSC 2.1 includes a number of algorithmic updates mainly aimed at reducing the number of quality assurance checks during processing. The algorithmic updates also make source fitting, photometry, and variability estimates more robust. Here is what is new in CSC 2.1:

Stacked Observation Detections Table Astrometric Correction:

CSC 2.1 observation stacks with sufficient CSC/Gaia matches (the majority of stacks) are now tied to the Gaia-CRF3 astrometric reference frame. The resulting coordinate shifts are typically 0.1-0.2 arcsec, but can be greater than 2 arcseconds in a very small percentage of the cases. CSC 2.1 sources that have had their astrometry updated will have non-zero or NULL values in the deltax and deltay columns in the Stack Detection Table. Details can be found in the Tying CSC Astrometry to the Gaia Reference Frame memo, and in the Stack Fine Astrometry Pipeline page.

Improved MCMC sampling for aperture photometry using pymc3:

For dim sources that fail to converge with the CSC 2.0 photometry algorithms, in CSC 2.1 we sample the marginalized posterior for the fluxes and hardness ratios using a more robust MCMC algorithm (NUTS). This improves our ability to provide well-constrained fluxes in the low-count regime. Details can be found in the Implementing pymc3 in CSC 2.1 Aperture Photometry memo.

Improved MLE fitting:

Bundles of sources for which overlapping detection regions prevented the automatic determination of their positions in CSC 2.0 are now fitted simultaneously using a model that accounts for multiplicity. This is described in the Revision to the MLE Algorithm for CSC Source Fitting memo.

Additional Spectral model:

For detections with a minimum of 150 net counts, their X-ray spectra are additionally fitted using an absorbed thermal plasma (APEC) model in addition to absorbed power-law, absorbed black body, and absorbed bremsstrahlung models.

Variability flag redefined to better support time-domain studies:

The variability flag (var_flag) at the master source and stacked observation detection levels are redefined to include both intra- and inter-observation variability (vs. only intra-observation variability in CSC 2.0), and the threshold for setting both variability flags and codes is changed from variability index 3 ("source may be variable") to variability index 6 ("source is definitely variable").