2014ApJ...789...98T

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 ≲ 0.7 keV coming from the disk and of temperature kT_s2 ≲ 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 Γ₂ is almost constant (Γ₂∼ 2) for all spectral states. In turn, Γ₁demonstrates a two-phase behavior with the spectral state: Γ₁is quasi-constant at the level Γ₁∼ 2 for the HB-NB and Γ₁ is less than 2, namely in the range of 1.3 < Γ₁< 2, when the source traces the FB. We also detect a decrease kT_s2from 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.