Abstract
Gamma-ray binaries are systems containing a massive star and a compact object that have been detected up to TeV energies. The high-energy emission could result from particle acceleration in the region where the stellar wind from the massive star interacts with the relativistic wind from a young pulsar. LS 5039 has the most compact orbit amongst gamma-ray binaries and its X-ray light curve shows a stable modulation synchronized with the orbital period. Photoelectric absorption of X-rays in the O star wind and occultation of the X-ray emitting region by the massive star can alter the X-ray light curve and spectrum along the orbit. Yet, the X-ray spectrum and light curve of LS 5039 do not show intrinsic absorption or X-ray eclipses. We study these effects in the framework of the pulsar wind scenario as a function of the binary inclination angle, the stellar wind mass-loss rate and the size of the X-ray emitter. An extended X-ray emission region ≳3R★ appears necessary to reconcile the pulsar wind scenario with observations.
Highlights
Gamma-ray binaries are systems containing a massive star and a compact object that emit most of their power at energies above 100 MeV
Electron acceleration could take place either in a relativistic jet or in the shocked wind of a young pulsar, where the shock results from the interaction between the pulsar wind and the stellar wind of the companion star
We find that the duration of the occultation is longest for binaries with circular orbits and where at all phases η0 > 1
Summary
Gamma-ray binaries are systems containing a massive star and a compact object that emit most of their power at energies above 100 MeV. X-ray absorption in the stellar wind and occultation of the X-ray emitting region by the massive star both result in orbital modulations with the correct phases for flux minimum and maximum. For LS 5039 there is no evidence for an absorption excess due to the stellar wind at any orbital phase, even at superior conjunction (Takahashi et al 2009) This has been used to argue that the X-ray emitting region must be far out from the system, where the column density of material crossed by the X-rays is small
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