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

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