From the middle of the 2000s, a new class of compact binary systems is
established, so-called gamma-ray binary systems.
They show non-thermal emission peaking above 1 MeV in their SEDs, which is
completely different from X-ray binaries.
This feature indicates that particle acceleration takes place in these
binaries, however, the physical mechanism of their activities is still
unclear.
In this talk, I will report our recent results of X-ray/gamma-ray
observations of the brightest gamma-ray binary system LS 5039:
1) LS 5039 has at least four spectral components from X-ray to TeV. The
dominant MeV gamma-ray component indicates a more efficient acceleration
than Fermi acceleration.
2) A sign of pulsation was found in the hard X-ray emission with $\sim 3 \sigma$.
If it is real, the period and its derivative suggest that the compact
object is a magnetar.
Based on these results, I also will explain a new acceleration scenario for
this object: LS 5039 contains a magnetar, and particles are accelerated via
magnetic reconnection close to the magnetar.
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