XID: Cross-association of ROSAT/Bright Source Catalog X-ray sources with USNO A-2 optical point sources

We quantitatively cross-associate the 18,811 ROSAT Bright Source Catalog (R ASS/BSC) X-ray sources with optical sources in the USNO A-2 catalog, calcul ating the probability of unique association (P-id) between each candidate w ithin 75 " of the X-ray source position, on the basis of optical magnitude and proximity. We present catalogs of RASS/BSC sources for which P-id > 98% , P-id > 90%, and P-id > 50%, which contain 2705, 5492, and 11,301 unique U SNO A-2 optical counterparts respectively down to the stated level of signi ficance. Together with identifications of objects not cataloged in USNO A-2 due to their high surface brightness (M31, M32,...) and optical pairs, we produced a total of 11,803 associations to a probability of P-id > 50%. We include in this catalog a list of objects in the SIMBAD database within 10 " of the USNO A-2 position, as an aid to identification and source classifi cation. This is the first RASS/BSC counterpart catalog which provides a pro bability of association between each X-ray source and counterpart, quantify ing the certainty of each individual association. The catalog is more usefu l than previous catalogs which either rely on plausibility arguments for as sociation or do not aid in selecting a counterpart between multiple off-ban d sources in the field. Sources of high probability of association can be s eparated out, to produce high-quality lists of classes (Seyfert 1/2s, QSOs, RS CVns) desired for targeted study, or for discovering new examples of kn own classes (or new classes altogether) through the spectroscopic classific ation of securely identified but unclassified USNO A-2 counterparts. Low P- id associations can be used for statistical studies and follow-on investiga tion-for example, performing follow-up spectroscopy of the many low-mass st ars to search for signatures of coronal emission, or to investigate the rel ationship between X-ray emission and classes of sources not previously well -studied for their X-ray emissions (such as pulsating variable stars). We f ind that a fraction similar to 65.8% of RASS/BSC sources have an identifiab le optical counterpart, down to the magnitude limit of the USNO A-2 catalog which could be identified by their spatial proximity and high optical brig htness.