Gaussian Process Inference of Stochastic Magneto-Active Dynamics and Viscosity in Swift J1727.8-1613

Abstract

Linking X-ray variability to the underlying magnetohydrodynamic (MHD) dynamics of black hole X-ray binaries remains challenging. We systematically investigate the stochastic and oscillatory variability of the black hole X-ray binary candidate Swift J1727.8$-$1613 during its 2023 outburst using Gaussian process (GP) regression applied to Insight-HXMT multi-band light curves. The variability is modeled with a physically motivated composite kernel comprising one stochastically driven damped simple harmonic oscillator (SHO) and two damped random walk (DRW) components. The SHO term robustly recovers quasi-periodic oscillations (QPOs) with frequencies $ν_0 \sim 0.07$--$5$ Hz, consistent with the fundamental Alfvén mode of a contracting magnetically confined disk--coronal cavity. The quality factor rises from $Q \sim 3$ to $Q \sim 10$, suggesting increasing coherence of the magnetic cavity. We also find an anti-correlation between QPO frequency and the short DRW damping timescale, supporting our proposed stochastic magneto-active dynamics scenario. Associating the short and long DRW timescales with the local turbulent turnover and thermal adjustment timescales, respectively, we infer an effective viscosity parameter of $α\approx 0.1$, supporting a strongly magnetized accretion flow. Strikingly, near the onset of relativistic jet ejection around MJD 60206, both relaxation timescales collapse toward the 0.1 s sampling limit, suggesting a rapid reorganization of the disk internal energy balance immediately before jet launching. Our results establish GP inference as a powerful route to connecting X-ray timing observables with the dynamical state of black hole accretion flows.

Figures (7)

• Figure 1: The HID of Swift J1727.8-1613 during the 2023 outburst. The hardness is defined as the LE count rate in the 2–10 keV energy range, while the hardness ratio is calculated as the ratio of the count rates in the 4–10 keV and 2–4 keV bands. Colored circles highlight specific observation IDs (ObsIDs) selected for detailed timing analysis.
• Figure 2: The light curves of Swift J1727.8–1613 observed by Insight-HXMT. The three panels from top to bottom display the count rates in the LE (2–10 keV), ME (10–35 keV), and HE (27–250 keV) bands, respectively. The gray circles and solid lines illustrate the overall temporal evolution. Specific observations selected for timing analysis are highlighted as colored circles, consistent with the epochs defined in Fig. \ref{['Fig1']}. The red vertical dashed line in the middle panel indicates the ME band peak flux observed at MJD 60184.12. Three vertical dot-dashed lines mark the timings associated with jet knot ejections: Jet Knot 3 (orange, MJD 60206.22), Jet Knot 2 (green, MJD 60206.36), and Jet Knot 1 (purple, MJD 60206.41).
• Figure 3: Top Left: The HE light curve (black points) fitted with the best-fit GP model (blue line) and the 1$\sigma$ uncertainty range. Top Right: Standardized residuals and their corresponding density distribution (right histogram). Middle Row: The ACF of the residuals (left) and squared residuals (right). Bottom Left: Corner plot showing the posterior probability distributions for the GP hyperparameters. Bottom Right: PSD decomposition. The total GP-recovered PSD (red) is compared with the Lomb-Scargle Periodogram (LSP, gray). The individual components include one SHO kernel (orange dashed) and two DRW kernels (green and purple dashed).
• Figure 4: Temporal evolution of the SHO model parameters. Different markers distinguish the energy bands: HE (pink circles), ME (purple squares), and LE (green triangles). The red vertical dashed line marks the peak of the ME X-ray flux. The vertical dotted lines on the right denote the ejection times of Knot 3 (orange), Knot 2 (green), and Knot 1 (purple).
• Figure 5: Temporal evolution of the two DRW model parameters. Same as Fig. \ref{['Fig4']}.