Exploring the connection between the stellar wind and the non-thermal emission in LS 5039
Abstract
New xmm observations have been performed around periastron and apastron passages in September 2005, during an epoch of presumably enhanced O star wind. Also, 2005 Chandra observations on LS 5039 are revisited. Moreover, a compilation of the Halpha EW and rxte/ASM X-ray count rate obtained since the 1990s is carried out, being both quantities compared with each other and also with historical radio data. xmm 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 a constant and with the interstellar value in both observations. The hardness ratio and the count rate seem uncorrelated at periastron and may be correlated at apastron. We find that LS 5039 was bright and hard in 2005 chandra observations. The ASM count rate shows changes by a ~80% on year timescales, and the Halpha EW shows yearly variations of a ~10%. Both quantities may be anticorrelated rather than correlated, unlike it was previously thought. At radio frequencies, the emission varies by ~20%, presenting some similarities with the X-rays. ASM, Halpha EW and radio data may hint to variability at orbital timescales. The low value and constancy of the N_H could imply that the X-ray emitter is located at >~10^12 cm from the compact object. We suggest that the non-thermal emission in LS 5039 is related to the jet and the stellar wind in a complex way and produced outside the system. However, the present data do not rule out the pulsar scenario.Keywords
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