Accretion Disk 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 disk with a focus on the disk normalization derived from the diskbb component, which serves as a proxy for the apparent inner disk radius. In the standard single Comptonization model, the disk normalization in the hard state is more than an order of magnitude lower than in the soft state ($\sim$0.3$\times$10$^3$ vs. $\sim$3.0$\times$10$^3$). This result contradicts the widely accepted view that the disk radius is smaller in the soft state than in the hard state. By incorporating an additional warm Comptonization component, the disk normalization in the hard state increases to values ($\gtrsim 10^4$) exceeding those in the soft state ($\sim 10^3$), yielding results consistent with a physically truncated, cooler accretion disk. 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.

Figures (9)

• Figure 1: The left panel displays the one-day binned light curve of GX 339-4, derived from MAXI data. The red triangles indicate the specific observations utilized in this analysis, with the corresponding dates detailed in Table \ref{['tab:table1']}. The right panel presents the Hardness–Intensity Diagram (HID) for GX 339-4, plotted using NICER data. The diagram shows the evolution of the hardness ratio (4-10 keV/2–4 keV) on the x-axis and the count rate in the 0.5–10 keV range on the y-axis. It highlights the simultaneous NICER, NuSTAR, and HXMT observations that are included in this study.
• Figure 2: The 10-second binned, barycentric-corrected light curves from NuSTAR (red, 3-80 keV), HXMT (LE, blue, 1-10 keV), and NICER (magenta, 2-12 keV) during two distinct states of the source. Left: Hard state (Epoch 1). Right: Soft state (Epoch 5).
• Figure 3: Simultaneous broadband spectra of GX 339$-$4 in the hard state (left; Epoch 1) and soft state (right; Epoch 5), fitted with the single-Comptonization model (Table 3). Data are shown from NICER (pink), NuSTAR/FPMA (red), NuSTAR/FPMB (black), Insight-HXMT/LE (green), Insight-HXMT/ME (blue), and Insight-HXMT/HE (cyan). The lower panels show the residuals, $(\mathrm{data}-\mathrm{model})/\sigma$, for the best-fitting models.
• Figure 4: Simultaneous broadband spectra of GX 339$-$4 in the hard state (left; Epoch 1) and soft state (right; Epoch 5), fitted with the best-fitting models reported in Table 4. Data are shown from NICER (pink), NuSTAR/FPMA (red), NuSTAR/FPMB (black), Insight-HXMT/LE (green), Insight-HXMT/ME (blue), and Insight-HXMT/HE (cyan). The lower panels show the residuals, for the best-fitting models.
• Figure 5: Unabsorbed model components for GX 339–4. The left panel shows the hard state (Epoch 1), and the right panel shows the soft state (Epoch 5). The red curve represents the total model, green indicates warm Comptonization, purple is the reflection component, and brown shows the relativistic disk emission. In the hard state, the blue curve includes both hot and warm Comptonization, while in the soft state, it represents the hot corona alone.