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XID: Cross-Association of ROSAT/Bright Source Catalog X-ray Sources with USNO A2 Optical Point Sources

Robert E. Rutledge, Robert J. Brunner, and Thomas A. Prince

Division of Physics, Mathematics and Astronomy MS 220-47, California Institute of Technology, Pasadena, CA 91107 rutledge@srl.caltech.edu, rb@astro.caltech.edu, prince@srl.caltech.edu

Carol Lonsdale

Infrared Processing and Analysis Center, Caltech/JPL, Pasadena, CA 91125, cjl@ipac.caltech.edu


We quantitatively cross-associate the 18811 ROSAT Bright Source Catalog (RASS/BSC) X-ray sources with optical sources in the \usno\ catalog, calculating the the probability of unique association (\pid) between each candidate within 75\arcsec\ of the X-ray source position, on the basis of optical magnitude and proximity. We present catalogs of RASS/BSC sources for which \pid$>$98\%, \pid$>$90\%, and \pid$>$50\%, which contain 2705, 5492, and 11301 unique \usno\ optical counterparts respectively down to the stated level of significance. Together with identifications of objects not cataloged in \usno\ due to their high surface brightness (M31, M32, ...) and optical pairs, we produced a total of 11803 associations to a probability of \pid$>$50\%. We include in this catalog a list of objects in the SIMBAD database within 10\arcsec\ of the \usno\ position, as an aid to identification and source classification. This is the first RASS/BSC counterpart catalog which provides a probability of association between each X-ray source and counterpart, quantifying the certainty of each individual association. The catalog is more useful than previous catalogs which either rely on plausibility arguments for association, or do not aid in selecting a counterpart between multiple off-band sources in the field. Sources of high probability of association can be separated out, to produce high-quality lists of classes (Seyfert 1/2s, QSOs, RS CVns) desired for targeted study, or for discovering new examples of known classes (or new classes altogether) through the spectroscopic classification of securely identified but unclassified \usno\ counterparts. Low \pid\ associations can be used for statistical studies and follow-on investigation -- for example, performing follow-up spectroscopy of the many low-mass stars to search for signatures of coronal emission, or to investigate the relationship between X-ray emission and classes of sources not previously well-studied for their X-ray emissions (such as pulsating variable stars). We find that a fraction $\sim$65.8\% of RASS/BSC sources have an identifiable optical counterpart, down to the magnitude limit of the \usno\ catalog which could be identified by their spatial proximity and high optical brightness.
Appendix: Tables of Cross-Identifications between the ROSAT/Bright Source Catalog and USNO-A2 Catalogs

These four tables make up the appendix for the paper: ``XID: Cross-Identifications of ROSAT/Bright Source Catalog Sources with USNO-A2 Optical Point Sources''. The production and meaning of all values in this table are given in that work, and are briefly described here:

Table             Content                 From Sec.  N_Obj
{tab:98}     greater  98\%                 3.1         2705
{tab:90}     90<  pid< 98 %          3.1         2787
{tab:50}     50<  pid< 90 %          3.1         5809
{tab:pair}   Optical Pairs  pid> 50%    3.2         472

The columns of each table (except Table~\ref{tab:pair}, see below) have the
following meanings:

column  & Heading             & Description
1       & 1RXS                & The ROSAT/BSC Name for the X-ray source
2       & PSPC c/s ($\sigma$) & RASS/BSC Catalog X-ray countrate, and 1$\sigma$ uncertainty
3       & \pid                & calculated fractional probability of unique association between the X-ray and USNO objects
4       & $B_{\rm USNO A2}$   & the \usno\ $B$ magnitude
5       & USNO A2             & the \usno\ source name/position (hhmmss.ss+/-ddmmss.s)
6       & SIMBAD crossID      & list of all objects $<$10\arcsec from the \usno\ position
7       & Type                & SIMBAD object population
8       & Class               & Classification of the SIMBAD object
9       & $B$ : $V$           & SIMBAD $B$ and $V$ magnitudes
10      & Comments            & Comment code

In Table~\ref{tab:pair}, the magnitudes and \usno\
designations of the two sources are separated by a backslash. Please
be aware of the points of caution regarding the ``Optical Pairs''
counterparts.  In column 7, the source types are often abbreviated
according to the following scheme, which closely follows the source
types used by SIMBAD:

Abbrev.  Meaning                    Abbrev.       Meaning                            Abbrev.    Meaning
$*$      Star                       HiPM*         High Proper-motion Star            SN         SuperNova
$**$     Stellar binary             HMXB          High Mass X-ray Binary             Spec. Bin. Spectroscopic Binary
$*$iC    Star in Cluster            IR            Infra-red object                   Sy1        Seyfert 1
$**$mul  multiple stellar system    Glob. Clust.  Globular Cluster                   Sy         Seyfert 2
$*$Neb   Star in Nebula             Gal.          Galaxy                             TT         T-Tauri type star
Clust.   Cluster                    GiG           Galaxy in Group of Galaxies        UV         Ultra-violet emission source
CV       Cataclysmic Variable       LMXB          Low Mass X-ray Binary              V*         Variable Star
Ceph.    Cephied Variable           LSBG          Low Surface Brightness Galaxy      WD         White Dwarf
DN       Dwarf Nova                 PN            Planetary Nebula                   WR *       Wolf-Rayet Star
Ecl.Bin. Eclipsing Binary (and type)Rad.          Radio source                       X          X-ray source
Em. *    Emission line star         Rot. Var. *   Rotationally Variable Star         YSO        Young Stellar Object

Additional lines are given for each SIMBAD object within 10\arcsec\ of
the USNO counterpart (20 \arcsec for the optical pairs) .  Note that
the counterpart for which the \pid\ applies is the \usno\ counterpart;
the SIMBAD identifications are listed to provide {\it possible}
identifications of this source. The SIMBAD $B$ and $V$ magnitudes
provide a point of comparison with the \usno\ $B$ magnitude which may
help in identifying the \usno\ object, however the \usno\ $B$
magnitudes are subject to certain systematic errors, and should be
viewed with caution.

Column 10 holds comment codes of which, at present, there is only one: P=SIMBAD
object has been previously identified as this RASS/BSC source. 

Catalog of RASS/BSC -- UNSO Associations,
with pid≥98%, and SIMBAD Sources within 10arcsec {tab:98}}
1RXS     PSPC c/s (sigma$)  pid   B_USNO A2  USNO A2  SIMBAD_crossID  Type   Class  $B$ : V$  Comments