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N. Shaposhnikov (USRA/GSFC), L. Titarchuk (NRL/GMU/CEOSR)
Recent studies have shown that strong correlations are observed between the low frequencies (1-10 Hz) of quasiperiodic oscillations (QPOs) and the spectral power law index of several Black Hole (BH) candidate sources, in low hard state, steep power-law (soft) state and in transition between these states. In the soft state this index-QPO frequency correlations show the saturation to the photon index \Gamma~2.7 at high values of the low frequency \nuL which was identified as a black hole signature. In this paper we argue that this saturation is not the case at least for NS source 4U 1728-34 for which the index \Gamma monotonically increases with \nuL to the values of 6 and higher. We reveal this effect analyzing almost all available data for 4U 1728-34 in the RXTE data archive. We show the spectral evolution of the Comptonized blackbody spectra when the source undergoes the transition from the hard to soft states. The hard state spectrum is a typical Comptonization spectrum of the soft photon radiation (from the disk and the NS surface) which \Gamma~1.8 whereas the soft state spectrum consists of two blackbody components which are only slightly Comptonized (the Comptonization Green's function index, \Gamma>6). The color disk and NS surface temperatures slightly decreases from 0.93 keV to 0.83 keV and from 2.9 keV to 2.2 keV respectively when the source undergoes the transition from the hard to soft states. Thus we can claim (as expected) that in the NS source the thermal equlibrium is established when the sources goes to the soft state whereas in BH sources because of BH horizon (the drain in system) the equilibrium is never established. The emergent spectrum, even in high/soft state, has a power law component. We also argue that the QPO low frequency \nuL is a fundamental frequency of the quasi-spherical component of the transition layer (corona)
frequency \nuSL is the frequency of oscillations of quasi-cylindrical configuration of the TL (presumably related to NS and disk magnetic field).
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.