| Author(s) and Co-Author(s) with Affiliation: V. P. Shyam Prakash(Space Astronomy Group, ISITE Campus, U. R. Rao Satellite Center, ISRO, Bengaluru 560037, India), Vivek K. Agrawal(Space Astronomy Group, ISITE Campus, U. R. Rao Satellite Center, ISRO, Bengaluru 560037, India), Rwitika Chatterjee(Space Astronomy Group, ISITE Campus, U. R. Rao Satellite Center, ISRO, Bengaluru 560037, India), Radhakrishna Vatedka(Space Astronomy Group, ISITE Campus, U. R. Rao Satellite Center, ISRO, Bengaluru 560037, India), Koushal Vadodariya(Space Astronomy Group, ISITE Campus, U. R. Rao Satellite Center, ISRO, Bengaluru 560037, India), A. M. Vinodkumar(Department of Physics, University of Calicut, Kerala 673635, India) |
| Abstract: In this work, we present a detailed spectral investigation of the neutron star low-mass X-ray binary, Scorpius X-1 using observations from the X-ray SPECtroscopy and Timing (XSPECT) payload onboard India’s first X-ray Polarimetry Satellite, XPoSat. Scorpius X-1 is the brightest X-ray source in the sky and the first X-ray source discovered. Owing to its extreme brightness in the soft X-ray band, studies at low energies have remained extremely challenging in the past. XSPECT, with fast read-out capability, enables high-quality low-energy X-ray observations of bright sources. During the observations the source trace a complete Z-track in its color–color diagram, including the horizontal, normal, and flaring branches. This represents the first resolved Z-track for Scorpius X-1 obtained using low-energy X-ray observations, highlighting the unique capability of XSPECT. We examine the evolution of spectral components along the Z-track to probe changes in the accretion flow geometry. The soft X-ray emission is well described by a multi-colour disc blackbody component, with the inner disc temperature varying between ∼0.6 and 0.8 keV and the hard X-ray emission using Comptonization models, yielding electron temperatures in the range ∼2.4–4.7 keV and optical depths of ∼5–14. Prominent iron Kα and Kβ emission lines are detected at ∼6.6 keV and ∼7.6 keV, respectively, indicating reflection from ionized material in the accretion environment. We observe a significant increase in both the disk and Comptonization fluxes during the flaring branch, accompanied by a rise in the neutron star blackbody and the inner disc temperature. Our results suggest that the Z-track evolution in Scorpius X-1 is primarily governed by variations in the coronal optical depth, Comptonization flux, disc flux, and inner disc temperature. No quasi-periodic oscillations are detected in any branch, implying a connection between QPO formation and higher-energy emission components. |
