2011ApJ...738..128S

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 high-luminosity states when the 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 Beppo SAX and RXTE satellites. We find that the X-ray broadband energy spectra of 4U 1728-34 during all spectral states can be modeled by a combination of a thermal (blackbody-like) component, a Comptonized component (which we herein denote as COMPTB), and a Gaussian component. Spectral analysis using this model provides evidence that the photon power-law index Γ is almost constant (Γ = 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 Γ 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. Moreover, this index stability effect now established for 4U 1728-34 during spectral evolution of the source was previously suggested for a number of other neutron binaries. This intrinsic property of the 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 the high state and saturates at high values of the mass accretion rate.