1997A&A...318..111M

We present 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=90deg, b=0deg and has an area of 64.5deg². A total of 128 soft X-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 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 1Gyr. 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 N_ns=10⁹ neutron stars in the Galaxy. If accretion on polar caps is the dominant mode then our upper limit may imply N_ns≃10⁸. Among the non coronal identifications are three white 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 P_orb≃12h.