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

TL;DR

This study uses the XSPECT instrument on XPoSat to chart Sco X-1 across the full Z-track in 0.8–15 keV, leveraging 1 ms readout to mitigate pile-up for a bright source. Spectral modeling with disk-related soft components and Comptonized hard continua (nthComp/Comptb) alongside Fe XXV lines reveals systematic, energy-dependent changes in disk temperature and radius, coronal optical depth, and seed-photon properties as the source moves HB→NB→FB, with flaring driving a pronounced rise in both disk and Comptonization flux. No QPOs are detected, but the Fe line complex evolves in tandem with the track, and the total luminosity remains dominated by Comptonization, reaching near or above the Eddington limit in parts of the track; these findings align with a disc–corona framework and with IXPE polarization results that favor disk-origin soft emission and boundary-layer–driven hard emission. Overall, the work provides a detailed, multi-model view of Sco X-1’s spectral and timing evolution, highlighting how changes in accretion rate and geometry govern the observed X-ray properties in this prototypical Z-source.

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 on-board Indias first X-ray Polarimetry Satellite (XPoSat), we have the capability to study the source despite its very high brightness, thanks to the fast (1 ms) readout 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 color-color diagram (CCD). Spectral studies indicate that the soft X-ray emission can be modeled using a multicolor disk component, with the inner disk temperature ranging from 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 2.4 to 4.7 keV and optical depth between 5 and 14. Additionally, we observe the presence of an iron K-alpha line at 6.6 keV and an iron K-beta line at 7.6 keV. Both spectral models suggest a steep rise in Comptonization flux as well as disk flux in the flaring branch. An increase in neutron star blackbody temperature and inner disk temperature are also observed during flaring. The Z-track is driven by changes in the optical depth of the corona, the Comptonization flux and the disk flux and the inner disk temperature. No quasi-periodic oscillations are detected in any branch, suggesting their association with the high-energy spectrum.

Figures (10)

• Figure 1: (Left) XSPECT light curve of Sco X-1 plotted for a bin size of 200 sec in the 0.8-15 keV energy band. Flaring activities are observed in the source during the observation in 60538-60545 (MJD). (Right) HID plotted using XSPECT observations of Sco X-1. Each data point represents a 200 s bins. Color code is given to each point to demonstrate the evolution of source through the Z track during the observation.
• Figure 2: HID plotted using XSPECT observations of Sco X-1 for a 200 s bins. The Z-track is divided into seven segments for spectral and timing studies.
• Figure 3: Comparison between the residuals obtained for (top) tbabs * (diskbb + nthComp) (middle) tbabs * (diskbb + nthComp + Gauss) and (bottom) tbabs * (diskbb + nthComp + Gauss + Gauss) showing the presence of Fe $K_{\alpha}$ and $K_{\beta}$ emission lines. The spectra from FoV 2$\times$2 and 3$\times$3 detectors are shown in black and red colors respectively.
• Figure 4: The 0.8-15.0
• Figure 5: The 0.8-15.0