Exploring the connection between the stellar wind and the non-thermal emission in LS 5039 (original) (raw)

2007, Astronomy and Astrophysics

Context. LS 5039 has been observed with several X-ray instruments so far showing quite steady emission in the long term and no signatures of accretion disk. The source also presents X-ray variability at orbital timescales in flux and photon index. The system harbors an O-type main sequence star with moderate mass loss. At present, the link between the X-rays and the stellar wind is unclear. Aims. We aim here at studying the X-ray fluxes, spectra, and absorption properties of LS 5039 at apastron and periastron passages during an epoch of enhanced stellar mass loss, and the long term evolution of the latter in connection with the X-ray fluxes. Methods. New XMM-Newton observations have been performed around periastron and apastron passages in September 2005, when the stellar wind activity was apparently higher. April 2005 Chandra observations on LS 5039 are also revisited. Moreover, a compilation of Hα EW data obtained since 1992, from which the stellar mass loss evolution can be approximately inferred, is carried out. Results. XMM-Newton observations show higher and harder emission around apastron than around periastron. No signatures of thermal emission or a reflection iron line indicating the presence of an accretion disk are found in the spectrum, and the hydrogen column density (N H ) is compatible with being the same in both observations and consistent with the interstellar value. 2005 Chandra observations show a hard X-ray spectrum, and possibly high fluxes, although pileup effects preclude obtaining conclusive results. The Hα EW shows yearly variations of a ∼ 10%, and does not seem to be correlated with X-ray fluxes obtained at similar phases, unlike it would be expected in the wind accretion scenario. Conclusions. 2005 XMM-Newton and Chandra observations are consistent with 2003 RXTE/PCA results, namely moderate flux and spectral variability at different orbital phases. The constancy of the N H seems to imply that either the X-ray emitter is located at > ∼ 10 12 cm from the compact object, or the real density in the system is 3 to 27 times smaller than the one predicted by a spherical symmetric wind model. We suggest that the multiwavelength non-thermal emission of LS 5039 is related to the observed extended radio jets and unlikely produced inside the binary system.