  Recent studies have revealed strong correlations between 1-10 Hz frequencies
of quasiperiodic oscillations (QPOs) and the spectral power law index of
several Black Hole (BH) candidate sources when seen in the low/hard state, the
steep power-law (soft) state, and in transition between these states. In the
soft state these index-QPO frequency correlations show a saturation of the
photon index about 2.7 at high values of the low frequency. This saturation
effect was previously identified as a black hole signature. In this paper we
argue that this saturation does not occur, at least for one neutron star (NS)
source 4U 1728-34, for which the index monotonically increases with the low
frequency to the values of 6 and higher. We base this conclusion on our
analysis about 1.5 Msec of RXTE archival data for 4U 1728-34. We reveal the
spectral evolution of the Comptonized blackbody spectra when the source
transitions from the hard to soft states. The hard state spectrum is a typical
thermal Comptonization spectrum of the soft photons which originate in the disk
and the NS outer photospheric layers. The hard state photon index is about 2.
The soft state spectrum consists of two blackbody components which are only
slightly Comptonized. Thus we can claim (as expected from theory) that in NS
sources thermal equilibrium is established for the soft state. To the contrary
in BH sources, the equilibrium is never established due to the presence of the
BH horizon. The emergent BH spectrum, even in the high/soft state, has a power
law component. We also show that the presence of Fe K_alpha emission-line
strengths, QPOs, and the link between them does not depend on radio flux in 4U
1728-34.
