Systematic Analysis of Changing-look AGN Variability Using ZTF Light Curves

TL;DR

This study investigates the variability of changing-look AGNs using ZTF light curves, revealing that their spectral changes are likely due to fluctuations in accretion rates rather than true type transitions.

Contribution

It provides a systematic analysis of CLAGN variability patterns and compares them with other AGN types, suggesting a critical accretion state causes spectral changes.

Findings

CLAGNs and EVQs differ from typical AGNs in luminosity and Eddington ratio.

On/off CLAGNs share similar variability patterns, distinct from EVQs and other AGNs.

Spectral changes in CLAGNs are likely due to accretion rate fluctuations, not genuine type transitions.

Abstract

Changing-look active galactic nuclei (CLAGNs) are a unique population of AGNs that exhibit the appearance (turn-on) or disappearance (turn-off) of broad emission lines. This study aims to explore the intrinsic mechanisms of CLAGNs by investigating their photometric variability using data from the Zwicky Transient Facility (ZTF), which has provided high-cadence observations over the past five years. By visual inspections, we construct a sample of 152 CLAGNs from the literature, all of which show spectral transitions and large optical variability in their ZTF light curves. By analyzing 90 of these CLAGNs and the control samples of Type 1 AGNs, Type 2 AGNs, and extremely variable quasars (EVQs), matched in redshift ($0.2<z<0.8$) and supermassive black hole mass, we compare the color variability, structure function (SF), and variability metric $\sigma_{\mathrm{QSO}}$, which quantifies how closely the light curves resemble a damped random walk (DRW) model. We find that while CLAGNs and EVQs differ from typical Type 1 and Type 2 AGNs in bolometric luminosity and Eddington ratio, the on/off-state CLAGNs share similar variability patterns with the overall CLAGN population, and distinct from EVQ, Type 1 and Type 2 AGNs. This suggests that 'on' and 'off' CLAGNs are not simply equivalent to Type 1 and Type 2 AGNs, respectively. Instead of undergoing genuine transitions between two AGN types, CLAGNs may inhabit a critical state where moderate fluctuations in accretion rate lead to the temporary spectral changes.