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

TL;DR

This study analyzes the photometric variability of changing-look AGNs (CLAGNs) using high-cadence ZTF light curves to test whether spectral transitions reflect simple type changes or a distinct accretion-state regime. By assembling 152 literature-identified CLAGNs (with $0.2<z<0.8$) and constructing matched control samples (EVQs, Type 1 and Type 2) in a 1:1:1:1 scheme, the authors compare color variability, structure-function parameters $A$ and $\gamma$, and DRW-based metrics $\sigma_{\mathrm{QSO}}$ and $\sigma_{\mathrm{var}}$. They find CLAGNs and EVQs exhibit stronger and more DRW-like variability than Type 1/Type 2 AGNs, but occupy an intermediate region in color–magnitude space and SF parameter space, with on/off states not matching a simple Type 1/Type 2 dichotomy. This supports a picture in which CLAGNs reside in a critical transitional state where moderate accretion-rate fluctuations drive temporary spectral changes, suggesting that CLAGNs do not strictly toggle between classical AGN types. The results highlight the importance of variability-based diagnostics for understanding accretion physics and the BLR’s response during spectral changes, and point toward future multiwavelength campaigns to test X-ray signatures and BLR evolution across CLAGN 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 $σ_{\mathrm{QSO}}$, 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.

Figures (10)

• Figure 1: Light curves of three CLAGN sources in different states: (a) turn-on and recently stable, (b) turn-on and recently unstable, (c) turn-off and recently stable. ZTF $g$- and $r$-band measurements are shown as blue and orange points, while SDSS measurements are shown as blue and orange triangles, respectively. The legend in (b) is the same for (a) and (c).
• Figure 2: Two-dimensional distributions of CLAGNs (a) and Type 2 AGNs (b) in the $\log M_{\rm BH}$–$z$ plane, and their logarithmic ratio (panel c). These distributions are used to construct a control sample of Type 2 AGNs matched to the CLAGNs in both dimensions via the probability-based sampling. Type 2 sample exhibits lower source density near all four corners of the 2D parameter space, resulting in larger sampling uncertainties and stronger deviations in the ratio.
• Figure 3: AGN properties for the CLAGN and control samples obtained from the SDSS catalog Shen2022. The skyblue and pink regions refer to Type 1 and Type 2 AGN samples respectively. The blue dashed line represents the distribution of EVQ samples and the red solid line shows the distribution of CLAGN samples.
• Figure 4: Color variability vs. magnitude variability for the CLAGN, SDSS J0040+1609, in the optical band. The gray dashed line represents the best-fit linear relation. The data confirm a bluer-when-brighter trend.
• Figure 5: Distributions of the $k$ values (slopes) from the linear fits to the color-magnitude variation data points for CLAGNs and the corresponding control samples. The lines have the same meaning as in Figure 1. Type 1 AGN and Type 2 AGN share different distribution while CLAGN located in the middle region.