  To understand the nature of the brightest gamma-ray binary system LS 5039,
hard X-ray data of the object, taken with the Suzaku and NuSTAR observatories
in 2007 and 2016, respectively, were analyzed. The two data sets jointly gave
tentative evidence for a hard X-ray periodicity, with a period of $\sim 9$ s
and a period increase rate by $\sim 3 \times 10^{-10}$ s s$^{-1}$. Therefore,
the compact object in LS 5039 is inferred to be a rotating neutron star, rather
than a black hole. Furthermore, several lines of arguments suggest that this
object has a magnetic field of several times $\sim 10^{10}$ T, two orders of
magnitude higher than those of typical neutron stars. The object is hence
suggested to be a magnetar, which would be the first to be found in a binary.
The results also suggest that the highly efficient particle acceleration
process, known to be operating in LS 5039, emerges through interactions between
dense stellar winds from the massive primary star, and ultra-strong magnetic
fields of the magnetar.
