Abstract AstroSat/LAXPC and SXT observed the persistent neutron star low-mass X-ray binary 4U 1820-30 between 2016 and 2022. During these observations, the hardness–intensity diagram and color–color diagram (CCD) indicated that the source was in the banana state. We divided the CCD into 11 segments for spectral and timing analyses. For each segment in the CCD, we modeled the spectral data using two distinct approaches over the 0.7–20.0 keV band. A combination of a multicolor disk component with an inner disk temperature of around 0.6 keV and Comptonized emission from the boundary layer/hot corona provided the best description of the X-ray spectral data of this source. The truncation radius was found to be in the range of ∼19–40 km. The Comptonized component has an optical depth in the range of ∼7–13, with electron temperature in the range of ∼2.5–3.8 keV. The optical depth of the corona varies significantly along the position on the CCD, while ∼80% of the X-ray flux comes from the Comptonized component. We discuss possible physical scenarios to explain the relationship between the spectral evolution and motion of the source along the CCD. The timing analysis revealed kHz quasi-periodic oscillation (QPO) peaks at ∼710 and ∼740 Hz in the lower left banana branch. An energy-dependent study indicates that these QPOs are stronger in the high-energy band.
Das et al. (Fri,) studied this question.