The Champ Extended Stellar Survey (chess): Photometric and Spectroscopic Properties of Serendipitously Detected Stellar X-ray Sources

We present 348 X-ray emitting stars identified from correlating the Extended Chandra Multiwavelength Project (ChaMP), a wide-area serendipitous survey based on archival X-ray images, with the Sloan Digital Sky Survey (SDSS). We use morphological star/galaxy separation, matching to an SDSS quasar catalog, an optical color-magnitude cut, and X-ray data quality tests to create our catalog, the ChaMP Extended Stellar Survey (ChESS), from a sample of 2121 matched ChaMP/SDSS sources. Our cuts retain 92% of the spectroscopically confirmed stars in the original sample while excluding 99.6% of the 684 spectroscopically confirmed extragalactic sources. Fewer than 3% of the sources in our final catalog are previously identified stellar X-ray emitters. For 42 catalog members, spectroscopic classifications are available in the literature. We present new spectral classifications and Hα measurements for an additional 79 stars. The catalog is dominated by main sequence stars; we estimate the fraction of giants in ChESS is ∼ 10%. We identify seven giant stars (including a possible Cepheid and an RR Lyrae star) as ChAMP sources, as well as three cataclysmic variables. We derive distances from ∼ 10− 2000 pc for the stars in our catalog using photometric parallax relations appropriate for dwarfs on the main sequence and calculate their X-ray and bolometric luminosities. These stars lie in a unique space in the LX–distance plane, filling the gap between the nearby stars identified as counterparts to sources in the ROSAT All-Sky Survey and the more distant stars detected in deep Chandra and XMM-Newton surveys. For 36 newly identified X-ray emitting M stars we calculate LHα/Lbol. LHα/Lbol and LX/Lbol are linearly related below LX/Lbol ∼ 3×10 , while LHα/Lbol appears to turn over at larger LX/Lbol values. Stars with reliable SDSS photometry have an ∼ 0.1 mag blue excess in u − g, likely due to increased chromospheric continuum emission. Photometric metallicity estimates suggest that the sample is evenly split between the young and old disk populations of the Galaxy; the lowest activity sources belong to the old disk population, a clear signature of the decay of magnetic activity with age. Future papers will present analyses of source variability and comparisons of this catalog to models of stellar activity in the Galactic disk. Subject headings: surveys — X-rays:stars — photometry:stars — spectroscopy:stars