An Update on Cross-matching CSC 2.0 with Other Major Catalogs by CXC

Cross-matching the second release of the Chandra Source Catalog (CSC 2.0) presents a number of unique challenges that need to be adequately addressed in order to achieve a high level of reliability and confidence in identifications that are made purely on the basis of spatial properties. The overriding consideration is that in cross-matching across disparate parts of the spectrum a reliable match probability needs to be obtained, since objects may appear dramatically differently in different parts of the spectrum, because of their overall physical and spectral properties and potential differences in obscuration. Providing a set of authoritative crossmatch catalogs for CSC 2.0 is an ongoing project at the Chandra X-ray Center (CXC). We have reported before on our approach to meeting the challenges. In short, it is not sufficient to base the determination of the spatial match probability solely on position uncertainties. One also needs to take into account the instrumental Point Spread Functions (PSF), the local density of sources, and, explicitly, the probability of ambiguous matches. This is especially important since Chandra's PSF varies very significantly over its field of view. That has led to our building a cross-match tool that is based on the Bayesian algorithms presented by Budavari, Heinis, and Szalay (ApJ 679, 301 and ApJ 705, 739), extended with the capability to allow for elliptical errors, PSFs, local source densities, and the explicit identification of ambiguous matches. This has necessitated determining composite PSFs for CSC 2.0 Master Sources, cross-matching individual CSC2.0 ensembles, proper thresholding of the match probabilities, and logic to identify ambiguities. We will present an update on the cross-match catalogs we have produced for CSC 2.0 with the SDSS, AllWISE, and GAIA catalogs. This work has been supported by NASA under contract NAS 8-03060 to the Smithsonian Astrophysical Observatory for operation of the Chandra X-ray Center.