2025A&A...704A.187Z

1ES 1927+654 has exhibited a spectroscopic changing-look transition following dramatic ultraviolet/optical (UV/optical) and X-ray variability in recent years. X-ray observations have revealed a rapid flux decline, when the hard X-ray power-law component disappeared, the soft thermal emission reached a minimum ∼150 days after the UV/optical peak, and both components reemerged with the source re-brightening. This extreme variability suggests the destruction and subsequent reformation of the inner disk and corona. Here, we report the discovery of quasiperiodic X-ray variability with a period of ∼12 days (significance > 3.2σ), which persisted for about 220 days, based on high-cadence monitoring during the inner disk-corona rebuilding phase. The signal is coherent with a very high quality factor of ∼58. We interpret this periodicity as a signature of radiation-pressure instability in the accretion disk, which occurs when the accretion rate and magnetic field strength reach appropriate values. This mechanism has been proposed as an explanation for quasiperiodic eruptions, a recently discovered intriguing phenomenon associated with galactic nuclei. Our findings provide a representative example of disk instability at moderate accretion rates. This phenomenon was long predicted by accretion theory, but rarely observed in active galactic nuclei (AGNs). Our research suggests that extreme events in AGNs, such as tidal disruption events, could serve as novel probes for testing and refining accretion theory.