  Gamma-ray loud X-ray binaries are binary systems that show non-thermal
broadband emission from radio to gamma rays. If the system comprises a massive
star and a young non-accreting pulsar, their winds will collide producing
broadband non-thermal emission, most likely originated in the shocked pulsar
wind. Thermal X-ray emission is expected from the shocked stellar wind, but
until now it has neither been detected nor studied in the context of gamma-ray
binaries. We present a semi-analytic model of the thermal X-ray emission from
the shocked stellar wind in pulsar gamma-ray binaries, and find that the
thermal X-ray emission increases monotonically with the pulsar spin-down
luminosity, reaching luminosities of the order of 10^33 erg/s. The lack of
thermal features in the X-ray spectrum of gamma-ray binaries can then be used
to constrain the properties of the pulsar and stellar winds. By fitting the
observed X-ray spectra of gamma-ray binaries with a source model composed of an
absorbed non-thermal power law and the computed thermal X-ray emission, we are
able to derive upper limits on the spin-down luminosity of the putative pulsar.
We applied this method to LS 5039, the only gamma-ray binary with a radial,
powerful wind, and obtain an upper limit on the pulsar spin-down luminosity of
~6x10^36 erg/s. Given the energetic constraints from its high-energy gamma-ray
emission, a non-thermal to spin-down luminosity ratio very close to unity may
be required.
