XMM-Newton observation of the enigmatic object WR 46^⋆

¹ Institut d’Astrophysique et de Géophysique, Université de Liège, allée du six août, 17, Bât. B5c, 4000 Liège, Belgium
e-mail: gosset@astro.ulg.ac.be
² Observatoire de Haute-Provence, 04870 St Michel l’Observatoire, France
³ Research and Scientific Support Department, ESTEC/ESA, PO Box 299, 2200, AG Noordwijk, The Netherlands
⁴ Sternberg Astronomical Institute, Moscow University, Universitetskij Prospect 13, Moscow 119992, Russia
⁵ European Space Agency, XMM-Newton Science Operations Centre, ESAC, Apartado 78, 28691 Madrid, Spain

Received: 16 May 2009
Accepted: 9 August 2010

Abstract

Aims. To further investigate the nature of the enigmatic object WR 46 and better understand the X-ray emission in massive stars and in their evolved descendants, we observed this variable object for more than two of its supposed cycles. The X-ray emission characteristics are appropriate indicators of the difference between a genuine Wolf-Rayet star and a specimen of a super soft source as sometimes suggested in the literature. The X-ray emission analysis might contribute to understanding the origin of the emitting plasma (intrinsically shocked wind, magnetically confined wind, colliding winds, and accretion onto a white dwarf or a more compact object) and to substantiating the decision about the exact nature of the star.

Methods. The X-ray observations of WR 46 were performed with the XMM-Newton facility over an effective exposure time of about 70 ks.

Results. Both the X-ray luminosity of WR 46, typical of a Wolf-Rayet star, and the existence of a relatively hard component (including the Fe-K line) rule out the possibility that WR 46 could be classified as a super soft source, and instead favour the Wolf-Rayet hypothesis. The X-ray emission of the star turns out to be variable below 0.5 keV but constant at higher energies. The soft variability is associated to the Wolf-Rayet wind, but revealing its deep origin necessitates additional investigations. It is the first time that such a variability is reported for a Wolf-Rayet star. Indeed, the X-ray emission exhibits a single-wave variation with a typical timescale of 7.9 h which could be related to the period observed in the visible domain both in radial velocities (single-wave) and in photometry (double-wave). The global X-ray emission seems to be dominated by lines and is closely reproduced by a three-temperature, optically thin, thermal plasma model. The derived values are 0.1−0.2 keV, 0.6 keV, and  ~4 keV, which indicates that a wide range of temperatures is actually present. The soft emission part could be related to a shocked-wind phenomenon. The hard tail of the spectrum cannot presently be explained by such an intrinsic phenomenon as a shocked wind and instead suggests there is a wind-wind collision zone, as does the relatively high L_X/L_bol ratio. We argue that this scenario implies the existence of an object farther away from the WN3 object than any possible companion in an orbit related to the short periodicity.