Evolution of the 2021 Outburst of GX 339-4 with AstroSat

TL;DR

This study analyzes the 2021 GX 339-4 outburst with AstroSat LAXPC and SXT, focusing on hard- and soft-intermediate states. Spectral fits using a disk blackbody plus Comptonization framework reveal a stable disc and a weakening corona as the source moves into SIMS, accompanied by a transition from type-C to type-B QPOs and eventual disappearance of QPOs. By applying a radiative-variation model to the rms and time-lag spectra, the authors find that simultaneous, first-order changes in the inner-disc temperature $kT_{in}$ and coronal heating rate $\dot H$, with a ~10 ms delay propagating to the disc normalization, can reproduce the observed energy-dependent variability, highlighting disc–corona coupling in state transitions. The results provide a coherent picture of QPO evolution and variability in BHXBs and point to the need to include reflection effects in future modeling for a more complete physical interpretation.

Abstract

We present a comprehensive study of the 2021 outburst of GX 339-4 using AstroSat observations in the hard-intermediate (HIMS) and soft-intermediate states (SIMS). Spectral and timing analyses across these states suggest that during the SIMS, unabsorbed flux (0.1-3 keV), inner disc temperature, and "apparent" inner disc radius do not change, suggesting the stability of the disc. In the SIMS, the photon index decreases from 2.1 to 1.7, indicating spectral hardening. The power density spectra (PDS) suggest the presence of quasi-periodic oscillations (QPOs) in the HIMS and SIMS. The QPO frequency evolves from 0.1 Hz to 0.2 Hz in the HIMS, and further to 5.7 Hz in the SIMS. We also observe a decrease in QPO frequency from 5.7 Hz to 4.5 Hz during the SIMS. We discuss the evolution of the QPO, fractional root mean square (rms) amplitude, and time-lag spectra. We discover that variations in disc normalization, disc temperature, and coronal heating rate can reproduce the observed rms and lag spectra with a time delay between them.

Figures (12)

• Figure 1: MAXI (2.0–20.0 keV) and Swift/BAT (15.0–50.0 keV) flux light curves of GX 339-4 during its 2021 outburst. The red, orange, and black vertical dashed lines, as well as the dashed grey region indicate the AstroSat observation period. The Swift/BAT flux is scaled by a factor of 25 for comparison.
• Figure 2: Hardness-Intensity Diagram (HID): The y-axis represents the MAXI flux in the energy range 2-20 keV, while the x-axis shows the HR2, defined as the ratio of Swift/BAT flux in the 15-50 keV range multiplied by 8, divided by the MAXI flux in the 2-20 keV range. The color gradient reflects the time evolution, with the corresponding colorbar displayed on the right side of the figure. Observations O1 (February 13, 2021), O2 (March 02, 2021), and O3 (March 05, 2021) are represented by red diamond, orange square, and black triangle
• Figure 3: Energy spectrum of observations O1 (Left), O2 (Middle), and O3 (Right), showing SXT (black squares) and LAXPC (orange circles) data points with corresponding error bars. The solid green lines represent the total model fits for each observation, fitted with Model 1S. The bottom panel displays the residuals ($\chi$) for each dataset. Individual model components are displayed: Gaussian (dashed red), and thcomp $\times$ diskbb (dotted blue).
• Figure 4: Energy Spectrum and Model Components. Left Panel: The upper section shows the observed SXT (black squares) and LAXPC (orange circles) spectra with error bars of segment 1 of O4, overlaid with the best-fit model (solid green line) for Model 1S:constant $\times$ tbabs $\times$ (gaussian + thcomp $\times$ diskbb). The lower section presents the residuals ($\chi$) as a function of energy. Right Panel: Individual model components are displayed: the total model (solid green), Gaussian (dashed red), and thcomp $\times$ diskbb (dotted blue).
• Figure 5: Evolution of the spectral parameters and fit statistics for observations O4$^*$ (red) and O4 (blue) as a function of MJD. The panels (top to bottom) show the photon index, covering fraction, inner disc temperature ($kT_{\mathrm{in}}$), apparent inner disc radius, and reduced $\chi^2$ from the spectral fits with Model 1S.