Gaussian-process Evidence for a Stochastic-variability Transition in the Recovering Corona of 1ES 1927+654

Abstract We investigate the stochastic X-ray variability of the changing-look active galactic nucleus 1ES 1927+654 during its 2018–2024 evolution, focusing on the recovery of the X-ray corona after its 2018 collapse. Using XMM-Newton EPIC-pn light curves in the 0.3–2.0 keV and 2.0–10.0 keV bands, we model the variability with Gaussian process (GP) covariance components including Matérn-3/2, damped random walk (DRW), stochastically driven damped simple harmonic oscillator (SHO), and white-noise terms. Bayesian model comparison reveals an X-ray stochastic-variability transition during the changing-look recovery phase. In the 2019 May 5 observation, the preferred covariance changes from a Matérn-3/2-like state to a DRW-like state within a single continuous exposure. A phenomenological gated-kernel estimate localizes this transition sharply in the hard band at t c ≃ 23.5 ks, while the soft band shows the same qualitative change over a broader interval. This transition occurs after the X-ray corona had reappeared but before the later pronounced hardening and brightening of the coronal emission, suggesting an early timing-domain signature of disk–corona reconfiguration. Phenomenologically, the dominant variability evolves from a smoother, finite-memory correlated process to a rougher, shorter-memory red-noise process. In the later 2022–2024 observations, SHO-like components associated with the known millihertz quasiperiodic oscillation (QPO) show increasing characteristic frequency and quality factor, indicating a faster and more coherent oscillatory component during the QPO-plus-jet phase. GP-based time-domain inference therefore provides a sensitive probe of stochastic-variability changes in recovering active galactic nucleus coronae.