The ROSAT galactic plane survey: analysis of a low latitude sample area in Cygnus

Wepresent the analysis of the point source content of a low galactic latitude region selected from the ROSAT all-sky survey. The test field is centered at l = 90◦, b = 0◦ and has an area of 64.5 deg2.A total of 128 softX-ray sources are detected above a maximum likelihood of 8. Catalogue searches and optical follow-up observations show that in this direction of the galactic plane, 85% of the sources brighter than 0.03 PSPC cts s−1 are identified with active coronae. F-K type stars represent 67% (± 13%) of the stellar identifications and M type stars account for 19% (± 6%). A small but significant number of X-ray sources are associated with A type stars on the basis of positional coincidence. These results together with those of similar optical campaigns demonstrate that the soft X-ray population of the Milky Way is largely dominated by active stars. We show that the density and distribution in flux and spectral type of the active coronae detected in X-rays are consistent with the picture drawn from current stellar population models and age dependent X-ray luminosity functions. The modelling of this population suggests that most of the stars detected by ROSAT in this direction are younger than 1 Gyr. This opens the possibility to extract in a novel way large samples of young stars from the ROSAT all-sky survey. The small number of unidentified sources at low X-ray flux put rather strong constraints on the hypothetical X-ray emission from old neutron stars accreting from the interstellar medium. Our observations clearly rule out models which assume no dynamical heating for this population and a total number of Nns = 109 neutron stars in the Galaxy. If accretion on polar caps is the dominant mode then our upper limit may imply Nns ≈ 108. Among the non coronal identifications are threewhite dwarfs, a Seyfert 1 active nucleus, two early type stars and one cataclysmic variable. We also report the discovery of a Me + WD close binary system with Porb ≈ 12 h.1 Send offprint requests to: C. Motch ? Partly based on observations obtained at theObservatoire deHauteProvence, CNRS, France 1 Tables 11 and 12 are also available in electronic form at the CDS via anonymous ftp cdsarc.u-strasbg.fr