We report on long-term stability of X-ray modulation apparently synchronized with an orbital period of 3.9 days in the gamma-ray binary LS 5039. Recent observations with the Suzaku satellite in the year 2007, which covered continuously more than one orbital period, have provided us with detailed characterization of X-ray flux and spectral shape as a function of orbital phase. Motivated by the results from Suzaku, we have re-analyzed the X-ray data obtained with ASCA, XMM-Newton, and Chandra between 1999 and 2005, to investigate long-term behavior of LS 5039 in the X-ray band. We found that the modulation curves in 1999--2007 are surprisingly stable. Even fine structures in the light curves such as spikes and dips are found to be quite similar from one orbit to another. The spectral characteristics observed in the past are consistent with those seen with Suzaku for some orbital phase segments. We suggest that magneto-hydrodynamical collisions between the relativistic outflow from a compact object and the stellar wind from the O star explain the clock-like non-thermal X-ray emission over eight years through remarkably stable production of high-energy particles near the binary system.
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