The Rate of Visual Binaries among the Brightest X-Ray Stars

Abstract

Using the Tycho Double Star Catalogue and the Hipparcos Catalogue, I compare the rates of occurrence of resolved visual doubles among the brightest X-ray stars from the ROSAT All-Sky Survey Bright Source Catalog (RASS-BSC) and generally in these two catalogs. A strong correlation between visual binarity and brightness in X-rays is found. The rate of binarity, quantified as the fraction of resolved components, is 7 times higher for the RASS-BSC stars than generally for all Tycho-2 stars. In order to eliminate the distance bias in the sample, I compare the rates in a distance-limited sample of Hipparcos stars within 50 pc. The comparison has shown that the fraction of visual and unresolved orbiting binaries among stars with counterparts in the RASS-BSC, hence bright in X-rays, is 2.4 times higher than among fainter X-ray stars, i.e., the ones whose X-ray brightness is below the limit of the RASS-BSC. This difference is not caused by the enhancement of the observed flux due to the combined component's radiation. Previously found in the nearby open clusters, the correlation between binarity and X-ray luminosity exists for nearby field stars as well. Possible implications of the discrepancy are discussed, including the binarity-age and binarity-rotation velocity relations. The H-R diagram of components of nearby visual binaries indicates that the red clump giants luminous in X-rays may be unresolved binaries.