The X-ray source content of the XMM-Newton Galactic plane survey^⋆

¹ CNRS, Université de Strasbourg, Observatoire Astronomique, 11 rue de l’Université, 67000 Strasbourg, France
e-mail: christian.motch@astro.unistra.fr
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

Abstract

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.