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 } \mathrm { erg s ^ { -1 } } .
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 \sim 6 \times 10 ^ { 36 } \mathrm { erg s ^ { -1 } } .
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 .