Exploring the connection between the stellar wind and the non-thermal emission in LS 5039 ^*,**

¹ Max Planck Institut für Kernphysik, Saupfercheckweg 1, Heidelberg 69117, Germany e-mail: vbosch@mpi-hd.mpg.de
² Observatoire Astronomique, rue de l'Université 11, 67000 Strasbourg, France e-mail: motch@astro.u-strasbg.fr
³ Departament d'Astronomia i Meteorologia, Universitat de Barcelona, c/Martí i Franquès 1, 08028 Barcelona, Catalonia, Spain e-mail: mribo@am.ub.es;jmparedes@ub.edu
⁴ Universidade de São Paulo, Instituto de Astronomia, Geofísica e Ciências Atmosféricas – IAG, Departamento de Astronomia, Rua do Matão 1226, 05508-900 São Paulo, Brazil e-mail: [rlopes;janot]@astro.iag.usp.br
⁵ Dpto. de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain e-mail: ignacio@dfists.ua.es
⁶ Istituto Nazionale di Astrofisica, via del Fosso del Cavaliere 100, 00133 Roma, Italy e-mail: andrea.martocchia@iasf-roma.inaf.it

Received: 19 March 2007
Accepted: 10 July 2007

Abstract

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 study 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 were performed around periastron and apastron passages in September 2005, when the stellar wind activity was apparently higher. April 2005 Chandra observations on LS 5039 were revisited. Moreover, a compilation of Hα EW data obtained since 1992, from which the stellar mass-loss evolution can be approximately inferred, was 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 conclusive results from being obtained. The Hα EW shows yearly variations of ∼10%, and does not seem to be correlated with X-ray fluxes obtained at similar phases, unlike what is 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 10¹² cm from the compact object, or the density in the system is 3 to 27 times smaller than that 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 is unlikely to be produced inside the binary system.