Accretion disc evolution in GX 339−4 across spectral states using NuSTAR, NICER, and Insight-HXMT observations

ABSTRACT We present a broadband spectral analysis of the black hole X-ray binary GX 339−4 during its 2021 outburst, covering both hard and soft spectral states. Using simultaneous observations from NuSTAR, NICER, and Insight-HXMT, we investigate the evolution of the accretion disc with a focus on the disc normalization derived from the diskbb component, which serves as a proxy for the apparent inner disc radius. In the standard single Comptonization model, the disc normalization in the hard state is more than an order of magnitude lower than in the soft state ($\sim 0.3 \times 10^3$ versus $\sim 3 \times 10^3$ ). This result contradicts the widely accepted view that the disc radius is smaller in the soft state than in the hard state. By incorporating an additional warm Comptonization component, the disc normalization in the hard state increases to values ($\gtrsim 10^4$) exceeding those in the soft state ($\sim 10^3$), yielding results consistent with a physically truncated, cooler accretion disc. The results of this work support the presence of a dual-corona geometry in the hard state, comprising both a hot, optically thin corona and a warm, optically thick corona, while the soft state spectrum is well described by a single hot Comptonization component alone. Our findings emphasize the importance of including a warm corona in hard-state spectra, as it leads to a more physically consistent picture of the accretion geometry across spectral states.