Spectral nature of Sco X-1 observed using the X-ray SPECtroscopy and Timing (XSPECT) payload onboard XPoSat

ABSTRACT Scorpius X-1 is the brightest and first discovered X-ray source in the sky. Studying this source in the low-energy band has been challenging in the past due to its high brightness. However, with the X-ray SPECtroscopy and Timing (XSPECT) payload onboard India’s first X-ray Polarimetry Satellite (XPoSat), we have the capability to study the source despite its very high brightness, due to the fast ($\sim$1 ms) read-out of the instrument. We investigate the evolution of the spectral and timing properties of Sco X-1 across the horizontal, normal, and flaring branch, as observed with XSPECT. We examine changes in the spectral parameters as a function of position on the colour–colour diagram. Spectral studies indicate that the soft X-ray emission can be modelled using a multicolour disc component, with the inner disc temperature ranging from $\sim$0.6 to 0.8 keV. The hard component is described by a Comptonized continuum using either the nthComp or Comptb model with electron temperatures from $\sim$2.4 to 4.7 keV and optical depth between $\sim$5 and $\sim$14. Additionally, we observe the presence of an iron $K\,\alpha$ line at $\sim$6.6 keV and an iron $K\,\beta$ line at $\sim$7.6 keV. Both spectral models suggest a steep rise in Comptonization flux as well as disc flux in the flaring branch. An increase in neutron star blackbody temperature and inner disc temperature is also observed during flaring. The Z-track is driven by changes in the optical depth of the corona, the Comptonization flux and the disc flux, and the inner disc temperature. No quasi-periodic oscillations are detected in any branch, suggesting their association with the high-energy spectrum.