Evolution of low frequency quasi-periodic oscillations in GX\,339--4 during its 2021 outburst using {\it AstroSat} data

Santanu Mondal, Anirudh Salgundi, Debjit Chatterjee, Arghajit Jana, H. -K. Chang, Sachindra Naik

Published 2023-03-07Version 1

The black hole X-ray binary GX\,339-4 showed an X-ray outburst during 2021. The {\it AstroSat} captured this outburst when the source entered into the intermediate flux state, while the count rate was declining. The source showed an alternating flux profile in a timescale of $\lesssim$100 ks, where the hard energy band was more variable than the soft band. The energy dependent timing study showed that the observed quasi-periodic oscillation (QPO) was prominent in the low energy bands, with its sub-harmonic and harmonic components. These components appear and disappear with time, which is observed in the orbit-wise QPO study. The rms spectra of all orbits exhibiting QPOs show its maximum amplitude $\sim$ 10 keV, may indicate that mostly 10 keV photons participated in originating QPOs. The energy-dependent time lag agrees its association and origin with the Comptonising corona. Finally, we discuss possible reasons behind the origin of different timing properties observed.