Detection of low-frequency breaks in power density spectrum of GX 339?4 in faint low/hard state observations using <i>AstroSat</i> data

ABSTRACT We present the spectro-timing analysis of black hole binary system GX 339?4 using AstroSat data extracted at beginning outbursts in 2017 and 2019. The joint spectral fitting Large Area X-ray Proportional Counter (LAXPC) Soft Telescope (SXT) revealed that source was a faint low/hard state for both 2019 observations, with nearly equal photon indexes ?1.57 ?1.58 Eddington ratios, L/LEdd, 0.0011 0.0046, respectively. addition reflection component into modelling improved fit (??2 ? 6 ??2 7 2019), pointing towards presence weak features arising due to irradiation accretion disc. power density spectrum consisted strong band-limited noise break low frequencies, described by combination few zero-centred Lorentzians. low-frequency ?6 mHz ?11 whose detection is validated results from independent detectors (LAXPCs SXT). frequency roughly consistent obtained earlier observations showed an evolution flux, which accordance truncated disc model. Associating viscous time-scale disc, we estimated truncation radius ?93 ?61 gravitational