I will present the detailed study of the 2021 outburst of the black hole
X-ray binary GX 339-4 using observations from AstroSat, focusing on the
hard-intermediate (HIMS) and soft-intermediate (SIMS) states.

Our analysis shows that during SIMS, the soft X-ray emission (0.1–3 keV),
the inner disc temperature, and the apparent disc radius remain unchanged,
suggesting that the accretion disc stays stable in this phase. At the same
time, the photon index decreases from about 2.1 to 1.7, pointing to a
gradual hardening of the spectrum.

From the timing perspective, the power density spectra (PDS) reveal the
presence of quasi-periodic oscillations (QPOs). These evolve in
frequency—from ~0.1 Hz to ~0.2 Hz during HIMS, then rising to ~5.7 Hz in
SIMS. Interestingly, we also observe the QPO frequency decreasing slightly
from 5.7 Hz to 4.5 Hz within SIMS.

We further studied the energy-dependent QPO properties such as rms and
time-lag. Our modeling suggests that energy-dependent rms and time-lag
properties of low-frequency QPOs in GX 339-4 are explained by stochastic
variations in inner disc temperature, disc normalization, and coronal
heating rate with ms delays, broadly supporting stochastic propagation
models.