Abstract:
The black hole X-ray binary GX 339-4 showed an X-ray outburst during 2021. The 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 time-scale of 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 nearly sub-harmonic
and harmonic components. These components appear and disappear with time, as observed in the orbit-wise QPO study. The
Q-value, fractional rms, and 4.8–5.6 Hz frequency infer the QPOs as type-B and the spectral state as soft intermediate. The
rms spectra of all orbits exhibiting QPOs show an increase in amplitude till ∼10 keV, beyond which it starts decreasing. This
may indicate that ∼10 keV photons contributed relatively more in QPOs than other energy band photons. The Lorentzian
normalization of the type-B QPO in different energy bands is consistent with the 10 keV peak. The energy-dependent time lag
is complex and could be associated with the Comptonizing corona or jet. Finally, we discuss possible reasons behind the origin
of different timing properties observed.
