Sudden emergence of a low-frequency hard X-ray lag in the Seyfert 1 galaxy Mrk 1044

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.