Accretion-Regulated Type Transitions in Changing-Look AGNs: Evidence from Two-Epoch Spectral Analysis

Abstract

The changing-look active galactic nucleus (CL-AGN), an extraordinary subpopulation of supermassive black holes, has attracted growing attention for understanding its nature. We present an analysis of the spectral properties of 203 low-redshift CL-AGNs ($z<0.35$) using two-epoch spectra from SDSS DR16 and DESI DR1 with time baseline ranging from $\sim$1000 to 8000 days, based on spectral fitting and decomposition. The sample consists of 11.3\% Type 1.0, 26.6\% Type 1.2, 43.1\% Type 1.5, and 19\% Type 1.8/2.0 AGNs. The total sample is divided into two datasets: Dataset A (110 objects) with minor spectral type variations, likely general AGN variability, and Dataset B (93 objects) showing significant type transitions and characteristic turn-on or turn-off behavior. Our results reveal clear optical continuum and emission-line variability, showing both bluer-when-brighter and redder-when-brighter trends. A strong correlation between the broad H$\beta$/[O~{\sc iii}] ratio and broad H$\alpha$ luminosity ($L_{\rm H\alpha}$), ${\rm log(H\beta/[O~III])}=(0.63\pm 0.07){\rm log}(L_{\rm H\alpha})-(26.49\pm2.96)\pm0.48$ for Dataset B, as well as the correlation between H$\beta$/[O~{\sc iii}] and Eddington ratio ($L_{\rm bol}/L_{\rm Edd}$), ${\rm log(H\beta/[O~III])}=(0.59\pm 0.08){\rm log}(L_{\rm bol}/L_{\rm Edd})+(1.02\pm0.15)\pm0.53$ for Dataset B, suggests that accretion rate variations drive changes in ionizing flux within the broad-line region, thereby triggering AGN type transitions. These findings underscore the critical role of supermassive black hole accretion processes in refining the AGN unification model. Future work should investigate potential connections between stellar evolution in outer accretion disk and the observed scatter in these correlations.