A research team from the Yunnan Observatories of the Chinese Academy of Sciences, together with international collaborators, have identified a nonmonotonic relationship between continuum lag and luminosity in changing-look (CL) active galactic nuclei (AGNs). The findings were recently published in The Astrophysical Journal.

AGNs rank among the most luminous objects in the universe, fueled by gas accretion onto supermassive black holes (SMBHs). The structure of the accretion disk is widely thought to depend on the accretion rate, closely linking AGN variability to SMBH growth and feedback processes. However, conventional AGN evolution unfolds over timescales of approximately 100,000 years—far longer than the duration of human astronomical observations. The discovery of CL AGNs, which exhibit rapid changes in accretion rate accompanied by the appearance or disappearance of broad emission lines, has opened a new window into accretion disk transitions.

Continuum reverberation mapping serves as a critical technique for measuring accretion disk size by tracking wavelength-dependent time lags in light curves. Nonetheless, observed lags frequently exceed the predictions of the standard thin-disk (SSD) model. This inconsistency may stem from diffuse continuum emission within the broad-line region or from limitations inherent to existing models. Most prior reverberation mapping studies based on broadband photometry have been restricted to a single luminosity state, hindering investigations of disk evolution and efforts to isolate diffuse continuum contamination.

To tackle these challenges, the research team has carried out a long-term spectroscopic monitoring campaign of the CL AGN NGC 4151 using the Lijiang 2.4-meter telescope since 2020. They acquired high-quality, pure continuum light curves spanning a wide wavelength range of 369–937 nm. NGC 4151 represents only the second AGN for which spectroscopic continuum reverberation mapping has been successfully conducted, and the first such measurement obtained using ground-based telescopes. In 2024, the campaign was expanded through coordinated observations with the Neil Gehrels Swift Observatory, enabling full coverage from the ultraviolet (192.8 nm) to the near-infrared.

Their analysis reveals that the measured time lags are 6.6 times greater than those predicted by the SSD model. Leveraging the strengths of spectroscopic data, the team detected clear signatures of diffuse continuum contamination in the lag spectrum, notably a prominent dip in time lags near the Balmer and Paschen jump regions.

Notably, the time lag does not increase monotonically with luminosity: it rises initially before declining at higher luminosities. This non-monotonic trend is statistically significant, inconsistent with SSD model expectations, and cannot be fully explained by diffuse continuum contamination alone. The researchers propose that the pattern may reflect an intrinsic Baldwin effect within the diffuse continuum, whereby the relative contribution of diffuse emission weakens compared to intrinsic continuum emission at higher luminosities, thereby reducing the observed lag.

These results provide critical observational constraints for the development of more sophisticated theoretical models and raise new questions regarding the precision of SMBH mass measurements.