X-RAY SPECTRAL AND TIMING BEHAVIOR OF SCORPIUS X-1. SPECTRAL HARDENING DURING THE FLARING BRANCH

We present an analysis of the spectral and timing properties of X-ray emission from the Z-source Sco X-1 during its evolution between the horizontal (HB) and flaring (FB) branches observed with the Rossi X-ray Timing Explorer during the 1996-2002 period. We find that the broadband (3-250 keV) energy spectra during all spectral states can be adequately reproduced by a model, consisting of two Comptonized components and an iron line. We suggest that the seed photons of kT(s1) less than or similar to 0.7 keV coming from the disk and of temperature kT(s2) less than or similar to 1.8 keV coming from the neutron star (NS) are each upscattered by hot electrons of a "Compton cloud" (hereafter the Comptb1 and Comptb2 components, respectively, which are associated with the similarly subscripted parameters). The photon power-law index Gamma(2) is almost constant (Gamma(2) similar to 2) for all spectral states. In turn, Gamma(1) demonstrates a two-phase behavior with the spectral state: Gamma(1) is quasi-constant at the level Gamma(1) similar to 2 for the HB-NB and Gamma(1) is less than 2, namely in the range of 1.3 < Gamma(1) < 2, when the source traces the FB. We also detect a decrease kT(s2) from 1.8 keV to 0.7 keV during the FB. We interpret this apparent quasi-stability of the indices during the HB-NB in the framework of the model in which the spectrum is determined by the Comptonized thermal components. This effect established for the Comptonized spectral components of the Z-source Sco X-1 is similar to what was previously found in the atoll sources 4U 1728-34, GX 3+1 and 4U 1820-30 and the Z-source GX 340+0 through all spectral states. However, we interpret the index reduction phase detected during the FB in Sco X-1 within the framework of a model in which the spectrum at the FB is determined by high radiation pressure from the NS surface.