Volume 523, November-December 2010
|Number of page(s)||39|
|Section||Stellar structure and evolution|
|Published online||18 November 2010|
The X-ray source content of the XMM-Newton Galactic plane survey⋆
CNRS, Université de Strasbourg, Observatoire Astronomique, 11 rue de
2 Department of Physics and Astronomy, University of Leicester, LE1 7RH, UK
3 Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, RH5 6 NT Surrey, UK
4 Instituto de Fìsica de Cantabria (CSIC-UC), 39005 Santander, Spain
5 Centre d’Étude Spatiale des Rayonnements, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
6 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
7 Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain
Received: 29 October 2009
Accepted: 2 July 2010
We report the results of an optical campaign carried out by the XMM-Newton Survey Science Centre with the specific goal of identifying the brightest X-ray sources in the XMM-Newton Galactic plane survey. In addition to photometric and spectroscopic observations obtained at the ESO-VLT and ESO-3.6 m, we used cross-correlations with the 2XMMi, USNO-B1.0, 2MASS, and GLIMPSE catalogues to advance the identification process. Active coronae account for 16 of the 30 positively or tentatively identified X-ray sources and exhibit the softest X-ray spectra. Many of the identified hard X-ray sources are associated with massive stars, possible members of binary systems and emitting at intermediate X-ray luminosities of 1032−34 erg s-1. Among these are (i) a very absorbed, likely hyper-luminous star with X-ray/optical spectra and luminosities comparable to those of η Carina; (ii) a new X-ray-selected WN8 Wolf-Rayet star in which most of the X-ray emission probably arises from wind collision in a binary; (iii) a new Be/X-ray star belonging to the growing class of γ-Cas analogues; and (iv) a possible supergiant X-ray binary of the kind discovered recently by INTEGRAL. One of the sources, XGPS-25, has a counterpart of moderate optical luminosity that exhibits HeII λ4686 and Bowen CIII-NIII emission lines, suggesting that this may be a quiescent or X-ray shielded low mass X-ray binary, although its X-ray properties might also be consistent with a rare kind of cataclysmic variable (CV). We also report the discovery of three new CVs, one of which is a likely magnetic system displaying strong X-ray variability. The soft (0.4–2.0 keV) band log N(>S) − log S curve is completely dominated by active stars in the flux range of 1 × 10-13 to 1 × 10-14 erg cm-2 s-1. Several active coronae are also detected above 2 keV suggesting that the population of RS CVn binaries contributes significantly to the hard X-ray source population. In total, we are able to identify a large fraction of the hard (2–10 keV) X-ray sources in the flux range of 1 × 10-12 to 1 × 10-13 erg cm-2 s-1 with Galactic objects at a rate consistent with what is expected for the Galactic contribution alone.
Key words: X-rays: binaries / X-rays: stars / surveys / binaries: close
Tables 9–11 and Figs. 32, 33 are only available in electronic form at http://www.aanda.org
© ESO, 2010
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