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We present an analysis of the spectral properties observed in X-rays fromNeutron Star X-ray binary 4U~1728-34 during transitions between the low and thehigh luminosity states when electron temperature kT_e of the Compton cloudmonotonically decreases from 15 to 2.5 keV. We analyze the transition episodesfrom this source observed with BeppoSAX and RXTE. We find that the X-raybroad-band energy spectra of 4U~1728-34 during all spectral states can bemodeled by a combination of a thermal (black body-like) component, aComptonized component (which we herein denote COMPTB) and a Gaussian component.Spectral analysis using this model evidences that the photon power-law indexGamma is almost constant (Gamma=1.99+/-0.02) when kT_e changes from 15 to 2.5keV during these spectral transitions. We explain this quasi-stability of theindex by the model in which the spectrum is dominated by the strong thermalComptonized component formed in the transition layer (TL) between the accretiondisk and neutron star surface. The index quasi-stability takes place when theenergy release in the TL is much higher than the flux coming to the TL from theaccretion disk. This intrinsic property of neutron star is fundamentallydifferent from that in black hole binary sources for which the indexmonotonically increases during spectral transition from the low state to highstate and saturates at high values of mass accretion rate.

On the Constancy of the Photon Index of X-ray spectra of 4U~1728-34 through all spectral states