Abstract
Abstract
Hard X-ray lags, where low-frequency variations in the hard X-ray band lag behind those in the soft band, have been detected in many active galactic nuclei (AGNs) and are generally attributed to the inward propagation of accretion-flow fluctuations through an extended corona. In a long XMM-Newton observation of the Seyfert 1 galaxy Mrk 1044, we found a remarkable transition in the lag behavior within a single exposure, while the X-ray flux and spectral shape remained largely unchanged. During the first 60 ks, no significant hard X-ray lag was detected, whereas in the subsequent 60 ks, a pronounced lag emerged. The lag was so prominent that a large-amplitude flux variation event during the lag-detected interval, characterized by a gradual dimming followed by recovery, produced a remarkable clockwise loop in the flux–softness diagram. The sudden appearance of the hard X-ray lag suggests that the X-ray corona underwent a rapid transition from a compact to an extended configuration. This scenario is further supported by two independent observational signatures: (1) the variability became noticeably smoother, with a redder power spectral density (PSD), during the lag-detected interval, and (2) the broad Fe K
α
line profile became narrower and the reflection continuum weaker. These findings highlight the diagnostic power of tracking rapid changes in hard X-ray lags for probing the physical structure and evolution of AGN coronae, and demonstrate that identifying prominent loops in the flux–softness diagram provides an effective way to locate intervals with significant hard X-ray lags.