Evolution of the 2021 outburst of GX 339–4 with  <i>AstroSat</i>

ABSTRACT We present a comprehensive study of the 2021 outburst of GX 339–4 using AstroSat observations in the hard-intermediate (HIMS) and soft-intermediate states (SIMS). Spectral and timing analyses across these states suggest that during the SIMS, unabsorbed flux (0.1–3 keV), inner disc temperature, and ‘apparent’ inner disc radius do not change, suggesting the stability of the disc. In the SIMS, the photon index decreases from ${\sim}2.1$ to ${\sim}1.7$, indicating spectral hardening. The power density spectra (PDS) suggest the presence of quasi-periodic oscillations (QPOs) in the HIMS and SIMS. The QPO frequency evolves from ${\sim}0.1$ to ${\sim}0.2$ Hz in the HIMS, and further to ${\sim}5.7$ Hz in the SIMS. We also observe a decrease in QPO frequency from ${\sim}5.7$ to ${\sim}4.5$ Hz during the SIMS. We discuss the evolution of the QPO, fractional root mean square (rms) amplitude, and time-lag spectra. We discover that variations in disc normalization, disc temperature, and coronal heating rate can reproduce the observed rms and lag spectra with a time delay between them.