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

Abstract

ABSTRACT We present the spectro-timing analysis of the black hole binary system GX 339−4 using AstroSat data extracted at the beginning of outbursts in 2017 and 2019. The joint spectral fitting of Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) data revealed that the source was in a faint low/hard state for both 2017 and 2019 observations, with nearly equal photon indexes of ∼1.57 and ∼1.58 and Eddington ratios, L/LEdd, of 0.0011 and 0.0046, respectively. The addition of a reflection component into the spectral modelling improved the fit (Δχ2 ≈ 6 for 2017 and Δχ2 ≈ 7 for 2019), pointing towards the presence of weak reflection features arising due to irradiation of the accretion disc. The power density spectrum consisted of strong band-limited noise with a break at low frequencies, described by a combination of a few zero-centred Lorentzians. The fitting revealed a low-frequency break at ∼6 mHz for 2017 and ∼11 mHz for 2019 observations, whose detection is validated by results from independent detectors (LAXPCs and SXT). The break frequency is roughly consistent with results obtained from earlier observations that showed an evolution of the frequency with flux, which is in accordance with the truncated disc model. Associating the break frequency with the viscous time-scale of the accretion disc, we estimated a truncation radius of ∼93 and ∼61 gravitational radius for 2017 and 2019 observations, respectively.