| Abstract : | We report detection of a characteristic low-frequency (mHz) break in the power density spectrum (PDS) of GX 339-4 in the faint low/hard state observations, which were extracted at the beginning of two outbursts of 2017 and 2019 with AstroSat. The energy spectrum of both observations could be described with an absorbed power-law of a nearly equal photon index of ∼1.57 and ∼1.58 and with Eddington ratio, L/L Edd, of 0.0011 and 0.0046, respectively. The X-ray continuum also showed weak reflection features, which were accounted with reflection components leading to improvement in χ 2 fit (∆χ 2 ≈ 6 for 2017 and ∆χ 2 ≈ 7 for 2019). With analysis of the timing properties, mHz break was observed in the PDS at ∼6 mHz for 2017 and at ∼11 mHz for 2019 observation, whose detection is validated by results from independent detectors (LAXPCs and SXT). These detections and the flux levels are consistent with earlier results of the source done in similar hard state. The physical description of the break frequency can be understood using the Disk truncated model, which associates this frequency with the viscous time scale at the truncation radius. Using this approach, we also constrain the truncation radius to be ∼93 and ∼61 gravitational radii for 2017 and 2019 observation, respectively. |
