We present an X-ray spectral analysis of the high-mass binary 4U 1700-37 during its hard-soft state evolution .
We use the Beppo SAX , Suzaku and RXTE ( Rossi X-ray Timing Explorer ) observations for this investigation .
We argue that the X-ray broad-band spectra during all spectral states can be adequately reproduced by a model , consisting of a low-temperature Blackbody component , two Comptonized components both due to the presence of a Compton cloud ( CC ) that up-scatters seed photons of T _ { s 1 } { { } _ { < } \atop { } ^ { \sim } } 1.4 keV , and T _ { s 2 } < 1 keV , and an iron-line component .
We find using this model that the photon power-law index is almost constant , \Gamma _ { 1 } \sim 2 for all spectral states . 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 while the 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 } is greater than 40 keV .
We explain this quasi-stability of \Gamma during most of hard-soft transitions of 4U 1700-37 in a framework of the model in which the resulting spectrum is described by two Comptonized components .
We find that these Comptonized spectral components of the HMXB 4U 1700-37 are similar to those previously found in NS sources .
This index dependence versus both mass accretion rate and kT _ { e } revealed in 4U 1700-37 is a universal observational evidence for the presence of a NS in 4U 1700-37 .