  We observed PSR B1259-63, a young non-accreting pulsar orbiting around a Be
star SS 2883, eight times with the Suzaku satellite in 2007, to characterize
the X-ray emission arising from the interaction between a pulsar relativistic
wind and Be star outflows. The X-ray spectra showed a featureless continuum in
0.6-10 keV, modeled by a power law with a wide range of photon index 1.3-1.8.
When combined with the Suzaku PIN detector which allowed spectral analysis in
the hard 15-50 keV band, X-ray spectra show a break at 5 keV in a certain
epoch. Regarding the system as a compactified pulsar wind nebula, in which e+e-
pairs are assumed to be accelerated at the inner shock front of the pulsar
wind, we attribute the X-ray spectral break to the low-energy cutoff of the
synchrotron radiation associated with the Lorentz factor of the relativistic
pulsar wind gamma_1 = 4x10^5. Our result indicates that Comptonization of
stellar photons by the unshocked pulsar wind will be accessible (or tightly
constrained) by observations with the Fermi Gamma-ray Space Telescope during
the next periastron passage. The PSR B1259-63 system allows us to probe the
fundamental properties of the pulsar wind by a direct means, being
complementary to the study of large-scale pulsar wind nebulae.
