Long term wide-band spectro-temporal studies of outbursting black hole candidate sources GX 339−4 and H 1743−322: AstroSat and NuSTAR results

We present a comprehensive spectrotemporal analyses of recurrent outbursting black hole sources GX 339$-$4 and H 1743$-$322 using available AstroSat and NuSTAR archival observations during 2016–2024. The nature of the outburst profiles of both sources are examined using long-term MAXI/GSC and Swift/BAT light curves, and failed as well as successful outbursts are classified. Wide-band (0.5–60 keV) spectral modelling with disc (diskbb) and Comptonized (Nthcomp) components indicates that GX 339$-$4 transits from hard (kTbb=0.12-0.77 keV, Γ ₙₜₕ=1.54-1.74, and Lbol=0.91-11.56 per cent LEdd) to soft state (kTᵢₙ~[≈ kTbb]=0.82-0.88 keV, Γ ₙₜₕ=1.46-3.26, Lbol=19.59-30.06~\ per\ centLEdd) via intermediate state (kTᵢₙ~[≈ kTbb]=0.56-0.88 keV, Γ ₙₜₕ=1.76-2.66, Lbol=2.90-16.09~\ per\ centLEdd), whereas H 1743$-$322 transits from quiescent to hard state (Γ ₙₜₕ=1.57-1.71, Lbol=2.08-3.48~\ per\ centLEdd). We observe type-B and type-C quasi-periodic oscillations (QPOs) in GX 339$-$4 with increasing frequencies ($0.10 - 5.37$ Hz) along with harmonics. For H 1743$-$322, prominent type-C QPOs are observed in frequency range 0.22–1.01 Hz along with distinct harmonics. Energy-dependent power spectral studies reveal that fundamental QPO and harmonics disappear beyond 20 keV in GX 339$-$4, whereas fundamental QPO in H 1743$-$322 persists up to 40 keV. We also observe that type-C QPOᵣₘₛ\, per\,cent decreases with energy for both sources although such variations appear marginal for type-B QPOs. Additionally, we report non-monotonic behaviour of photon index with plasma temperature and detection of annihilation line. Finally, we discuss the relevance of the observational findings in the context of accretion dynamics around black hole binaries.