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We present an X-ray spectral analysis of the high-mass binary 4U~1700-37during its hard-soft state evolution. We use the BeppoSAX, Suzaku and RXTE(Rossi X-ray Timing Explorer), Suzaku and BeppoSAX observations for thisinvestigation. We argue that the X-ray broad-band spectra during all spectralstates can be adequately reproduced by a model, consisting of a low-temperatureBlackbody component, two Comptonized components both due to the presence of aCompton cloud (CC) that up-scatters seed photons of $T_{s1}$~< 1.4 keV, and$T_{s2}<$1 keV, and an iron-line component. We find using this model that thephoton power-law index is almost constant, $\Gamma_{1}\sim 2$ for all spectralstates. However, $\Gamma_{2}$ shows a behavior depending on the spectral state.Namely, $\Gamma_{2}$ is quasi-constant at the level of $\Gamma_{2}\sim 2$ whilethe CC plasma temperature $kT^{(2)}_e$ is less than 40 keV; on the other hand,$\Gamma_{2}$ is in the range of $1.3<\Gamma_{2}<2$, when $kT^{(2)}_e$ isgreater than 40 keV. We explain this quasi-stability of $\Gamma$ during most ofhard-soft transitions of 4U~1700-37 in a framework of the model in which theresulting spectrum is described by two Comptonized components. We find thatthese Comptonized spectral components of the HMXB 4U~1700-37 are similar tothose previously found in NS sources. This index dependence versus both massaccretion rate and $kT_e$ revealed in 4U~1700-37 is a universal observationalevidence for the presence of a NS in 4U 1700-37.

X-Ray Spectra of The High-Mass X-RAY Binary 4U~1700-37 using BeppoSAX, Suzaku and RXTE Observations