New massive X-ray binary candidates from the ROSAT Galactic Plane Survey I - Results from a cross-correlation with OB star catalogues

Abstract

We report the discovery of several new OB/X-ray accreting binary candidates. These massive systems were found by cross-correlating in position SIMBAD OB star catalogues with the part of the ROSAT all-sky survey located at low galactic latitudes (|b| < 20d) and selecting the early type stars which apparently displayed the most significant excess of X-ray emission over the `normal' stellar level. The present search is restricted to stars earlier than B6 and X-ray luminosities > 10^31 erg/s. Follow-up optical and X-ray observations allowed to remove misidentified OB stars and spurious matches with interloper X-ray emitters (mostly active coronae) leaving five very likely new massive X-ray binaries. These new candidates have 0.1-2.4 keV un-absorbed luminosities > 10^33 erg/s indicating an accreting neutron star or black hole. On the average their soft X-ray luminosities are comparable to those observed from hard X-ray transients in quiescence or from persistent low luminosity Be/X-ray sources. Two additional X-ray binary candidates require further confirmation of their X-ray excess. Their lower soft X-ray luminosities (1-5 10^32 erg/s) could qualify them as Be + accreting white dwarf systems.