CSC Crossmatches

Introduction

This page contains brief descriptions of the cross match tables between the Chandra Source Catalog and other catalogs across the electromagnetic spectrum. The CSC sources are extracted from the Master Sources Table. For each counterpart, the match probability (between 0.0 and 1.0) and separation (in arcsec) are given so that users may apply their own selection criteria appropriate for their goals. Starting with the cross matches for CSC version 2.1.1 the tables contain two additional items to aid users in applying more finely tuned selection criteria: normalized separation (separation divided by match σ) and match class (d, l, c, k, r; see below).

CSC 2.1.1 — SDSS DR18 Crossmatches

All cross matches have been performed with Xmatch employing the Bayesian method developed by Budavari & Szalay (2008, ApJ 679, 301) as implemented and extended by A. Rots (2025). The Xmatch code takes into account local source density as well as both error ellipses and raw-size ellipses of the sources. For each cross-matched source pair the match probability (between 0.0 and 1.0), separation (in arcsec), normalized separation (separation divided by match σ uncertainty), and match class (see below) are provided. The details on these columns are provided in the Xmatch quick summary. The main match tables are provided in four different formats: TSV, CSV, FITS, and VOTABLE. In addition, ambiguous matches are provided in four separate tables: ambiguous pairs based on error ellipse matches, ambiguous pairs based on raw size matches, CSC sources with ambiguous matches, and counterpart sources with ambiguous matches.

The cross matches are performed separately for each CSC Field: these are areas defined by the fields-of-view of observations covering a connected contiguous region on the sky. For CSC 2.1 there are 6019 such Fields, of which 5997 contain one or more master sources. It should be noted that match probabilities within each Field are (obviously) comparable, but their distribution may differ between Fields due to differences in source density and Bayes Factor distribution. This effect has been taken care of by applying a self-consistent probability threshold for each Field.

To aid users in selecting cross matches for their use, we have introduced five cross match classes. Users are advised to carefully consider which of the five cross match classes they want to include in order to obtain their desired level of robustness:

* 1.7σ corresponds to 76% confidence, 3.4σ to 99.7% confidence level; i.e., the likelihood of a match beyond 3.4σ is 0.3%.

** This means that the match has a high probability, but may be contaminated by another match with a distinctly lower but reasonable probability.

Cross Match Class
d	definite match, with a separation less than 1.7σ *
l	likely match, with a separation between 1.7σ and 3.4σ *
c	definite match, but with a potential ambiguity **
k	likely match, but with a potential ambiguity **
r	potential match based on raw source size; not robust

For more information, see the Xmatch quick summary or full documentation.

For more information, see the CSC 2.1–SDSS Primary Crossmatch Column Descriptions table for a description of the columns in the main crossmatch tables.

PRELIMINARY CSC 2.1 Database Crossmatches for SDSS-V Target Selection and SDSS Spectra

Using the NWAY algorithm, we have crossmatched the X-ray sources extracted from the CSC 2.1 production database on 14 November 2022 (389k sources) to four optical and near-IR catalogs—Gaia DR3, Legacy Survey DR10, PanSTARRS-1, and 2MASS— finding counterparts for 229k sources. The resulting matched catalog lists a subset of X-ray information, catalog IDs, positions, and magnitudes for all optical/IR matches. Cross-matching this table with SDSS DR17 spectroscopy yields about 17k objects for which SDSS spectra, classifications, and radial velocities are given. We present a README file describing the catalog, and the cross-match catalog itself in FITS and CSV format.

A crossmatched table of 188k sources has been submitted to the SDSS-V project as potential targets for spectroscopy.

CSC 2.0-SDSS DR15 Crossmatches

All crossmatches have been performed with a Bayesian method developed by Budavari & Szalay (2008) as implemented and extended by A. Rots (2020) in the Xmatch code that takes into account local source density as well as both error ellipses and raw-size ellipses of the sources. For each crossmatched source pair the match probability (between 0.0 and 1.0), match type (E when positional error ellipse is used, R if source raw size is used) and match grade (D for definite matches, L for likely matches) are provided. The details on these columns are provided in the Xmatch quick summary. In addition, ambiguous matches are provided in separate tables.