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 Rossi X-ray Timing Explorer 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 Γ 2 is almost constant (Γ 2 ∼ 2) for all spectral states. In turn, Γ 1 demonstrates a twophase behavior with the spectral state: Γ 1 is quasi-constant at the level Γ 1 ∼ 2 for the HB−NB and Γ 1 is less than 2, namely in the range of 1.3 < Γ 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 -2found 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.

KeplerK2 observations of Sco X-1: orbital modulations and correlations withFermiGBM and MAXI

“…The transitions, therefore, do not correspond to significant changes in either the X-ray intensity or the X-ray spectrum, so cannot be explained by either luminosity or spectral shape changes. Even if spectral changes did come into play, we have shown in Section 8 that the Xray spectral changes between high and low state are quite modest, at least from 2-40 keV, consistent with findings of Church et al (2012) and Titarchuk, Seifina, & Shrader (2014). It thus seems that we must look to changes in the illumination geometry to explain the optical state changes.…”

“…This would be consistent with several more recent X-ray studies such as Church, Halai, & Ba lucińska-Church (2006) and Church et al (2012), which locate the point of highest accretion rate at the hard apex where X-ray flux is maximized. Titarchuk, Seifina, & Shrader (2014) also suggest that the X-ray flux should trace the accretion rate well. We will examine this possibility further in the following section.…”

“…Furthermore, as Sco X-1 moves from the soft apex up the normal branch, the optical flux continues to decrease even while the X-ray flux is increasing; this is the X-ray to optical anti-correlation. Detailed spectral modeling by Church et al (2012) suggests that movement away from the soft apex along the FB and the NB both involve increasing X-ray luminosity, and this conclusion is supported by Titarchuk, Seifina, & Shrader (2014). The decrease in optical flux cannot then simply represent reduced X-ray luminosity, so if it arises by irradiation this must reflect a decreased irradiation efficiency in the low state (c.f.…”

“…It is seen that dramatic changes in the quasi-periodic oscillation frequencies can occur with negligible movement in the Z diagram (Dieters & van der Klis 2000), and that the bottom NB and bottom FB have nearly identical energy spectra but quite different time variability properties (Titarchuk, Seifina, & Shrader 2014). As a result distinctly different normal and flaring branch states can coexist at the soft apex for virtually the same X-ray intensity and colours.…”

“…A variety of behaviours could produce the observed signatures. If there is a thick torus in the inner disc, for example the transition region of Titarchuk, Seifina, & Shrader (2014), then decreasing the geometrical thickness of the torus would increase disc irradiation. If there is clumpy absorbing material above the disc, then decreasing the covering factor would increase irradiation of both disc and companion.…”