X-ray spectral and timing behavior of Scorpius X-1. Spectral hardening during the flaring branch

Abstract

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 RXTE during the 1996 -- 2002 period. We find that the broad-band (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" (herein Comptb1 and Comptb2 components respectively with which are associated similarly subscripted parameters). The photon power-law index Gamma_{2} is almost constant (Gamma_{2}~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}~2 for the HB-NB and Gamma_{1} is less than 2, namely in the range of 1.3<Gamma_1<2, when 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 established for the Comptonized spectral components of the Z-source Sco~X-1 is similar to that 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.