A new study led by researchers from the Yunnan Observatories of the Chinese Academy of Sciences, in collaboration with partners, has yielded new statistical evidence on the behavior of changing-look active galactic nuclei (CL-AGNs).

By analyzing two-epoch spectra of 203 such objects, the research team has statistically confirmed that variations in black hole accretion rates directly regulate ionizing flux in the broad-line region—triggering a complete spectral transition from type 1.0 to type 2.0 in active galactic nuclei (AGNs).

The study was recently published in The Astrophysical Journal.

The findings reveal that AGN spectral evolution is universally linked to the accretion process of supermassive black holes. Through regression analysis, the team achieved a parametric description of spectral type transitions—challenging the unified model, which relies solely on viewing angle effects, and explicitly incorporating the historical evolution of accretion processes into the core framework of AGN classification.

More than 1.5 million AGNs have been observed across various cosmic epochs, making them critical probes for studying the structure and evolution of the universe. While the unified model attributes AGN types to viewing angles and their energy to the accretion process of supermassive black holes at galactic centers, the discovery of CL-AGNs has posed a key challenge: These objects exhibit extreme changes, with broad emission lines appearing or disappearing over years to decades—a phenomenon not explained by static models.

To address this, the research team analyzed data from the Sloan Digital Sky Survey (SDSS DR16) and the Dark Energy Spectroscopic Instrument (DESI DR1). They selected a sample of 203 CL-AGNs with redshifts below 0.35—ensuring coverage of both Hα and Hβ broad emission lines. Through precise spectral fitting and parameter measurement, the sample was divided into two categories: Data Set A (110 sources), which showed only minor spectral type fluctuations (likely reflecting variability in normal AGNs), and Data Set B (93 sources), which exhibited significant "on-off" behavior characteristic of typical CL-AGNs.

Bayesian linear regression analysis identified two core correlations: AGN spectral type changes are positively correlated with both Hα broad emission line luminosity and the Eddington ratio (the ratio of bolometric luminosity to Eddington luminosity). These correlations were more pronounced in Data Set B. The results indicate that as accretion rates increase, ionizing photon flux in the broad-line region intensifies—transforming an AGN from type 2.0 (no broad lines) to type 1.0 (prominent broad lines). The reverse process occurs as accretion rates decrease.

The study suggests that CL-AGN behavior is closely tied to accretion flow structure. For example, when the accretion rate drops below a critical threshold (with an Eddington ratio near 0.02), a standard thin disk may transition to a radiatively inefficient accretion flow (ADAF)—causing a sharp decline in ionizing radiation. While previous case studies of Mrk 1018 observed a complete cycle transition from type 2.0 to 1.0 accompanied by accretion rate changes (Eddington ratio rising from 2×10^-5 to 0.02), this sample analysis further verifies the universality of this mechanism.

"The accretion process of black holes is like the 'heartbeat' of AGNs," said Associate Professor LU Kaixing, corresponding author of the study. "Its fluctuations are sufficient to alter the observable characteristics of the entire system. Through sample statistics and individual case tracking, the series of works related to this research have, for the first time, discovered that the optical observation features such as the type transformation of AGN is directly related to the accretion physics."