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Astron. Astrophys. 318, 111-133 (1997)

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

C. Motch 1, 2, P. Guillout 1, F. Haberl 2, M. Pakull 1, W. Pietsch 2 and K. Reinsch 3

1 Observatoire Astronomique, UA 1280 CNRS, 11 rue de l'Université, F-67000 Strasbourg, France
2 Max-Planck-Institut für Extraterrestrische Physik, D-85748 Garching bei München, Germany
3 Sternwarte, Geismarlandstrasse 11, D-37083 Göttingen, Germany

Received 20 March 1996 / Accepted 22 April 1996

Abstract

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 = [FORMULA], b = [FORMULA] and has an area of [FORMULA]. 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-1 are identified with active coronae. F-K type stars represent 67% ([FORMULA] 13%) of the stellar identifications and M type stars account for 19% ([FORMULA] 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 [FORMULA] = 109 neutron stars in the Galaxy. If accretion on polar caps is the dominant mode then our upper limit may imply [FORMULA] [FORMULA] 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 [FORMULA] 12 h. 1

Key words: X-ray general – X-ray stars – stars: activity – stars: neutron – stars: statistics

* Partly based on observations obtained at the Observatoire de Haute-Provence, CNRS, France

Send offprint requests to: C. Motch

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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