Gaussian-process Evidence for a Stochastic-variability Transition in the Recovering Corona of 1ES 1927+654
- 1Yunnan University
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.
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BibTeX
@article{dong2026gaussianprocess,
title = {{Gaussian-process Evidence for a Stochastic-variability Transition in the Recovering Corona of 1ES 1927+654}},
author = {Lijuan Dong and Dahai Yan},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/aeaa4e},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aeaa4e},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Gaussian-process Evidence for a Stochastic-variability Transition in the Recovering Corona of 1ES 1927+654
AU - Lijuan Dong
AU - Dahai Yan
PY - 2026
DA - 2026/10/28
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aeaa4e
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aeaa4e
AB - 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.
SN - 1538-4357
SN - 0004-637X
ER -