Search for low-mass companions to X-ray emitting A-type stars

There is no obvious theory that would explain X-ray emission from main-sequence A-type stars. Therefore, the X-ray emission identified with these stars on basis of low spatial resolution X-ray observations is usually attributed to magnetic activity from unknown late-type companion stars. We systematically study the literature and public 2 MASS data in search for binaries among A stars. This way, we identify new candidate counterparts for the X-ray sources in at least 60 % of the A-type stars claimed to be detected in the ROSAT All-Sky Survey. There is marginal evidence that this fraction decreases from early to late A spectral type, possibly indicating the onset of intrinsic X-ray emission for the latest A stars which may have deep enough convection zones to support magnetic activity.