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New massive X-ray binary candidates from the ROSAT Galactic Plane Survey *
I. Results from a cross-correlation with OB star catalogues
C. Motch 1, 2,
F. Haberl 2,
K. Dennerl 2,
M. Pakull 1 and
E. Janot-Pacheco 3
Received 18 April 1995 / Accepted 18 October 1996
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 ( ) 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 1031 erg s-1. 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: the O7 star LS 5039 and the Be stars BSD 24- 491, LS 992, LS 1698 and LS I +61 235. This latter source was already mentioned in an earlier paper. LS 1698 is the probable optical counterpart of the hard X-ray transient 4U 1036-56.
These new candidates have 0.1-2.4 keV un-absorbed luminosities 2 1033 erg s-1 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. The four Be stars have Balmer emission slightly less intense than previously known systems showing strong outbursts. This suggests that the relative weakness of the circumstellar envelope may explain the low luminosities to some extent.
Two additional X-ray binary candidates, HD 161103 and SAO 49725 require further confirmation of their X-ray excess. Their lower soft X-ray luminosities (1-5 1032 erg s-1) could qualify them as Be + accreting white dwarf systems.
Four other B stars in the Orion and Canis Major OB associations, HD 38087, HD 38023, HD 36262 and HD 53339 exhibit X-ray flux excesses in the range 2-7 1031 erg s-1 whose origin is unclear. Finally very soft X-ray emission was detected from HR 2875 suggesting the presence of a non-accreting white dwarf companion to the B5 star.
Key words: X-ray: stars stars: early-type stars: emission-line, Be, neutron stars, white dwarfs
* Partly based on observations obtained at the Observatoire de Haute-Provence, CNRS, France, on observations collected at the European Southern Observatory La Silla, Chile, with the 2.2 m telescope of the Max-Planck-Society and on observations acquired at the Laboratorio Nacional de Astrofisica, CNPq, Brazil
Send offprint requests to: C. Motch
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998