Timescale-dependent Optical Variability of Turn-on Changing-look Active Galactic Nuclei

Abstract

Changing-look active galactic nuclei (CL AGNs) provide a valuable opportunity to study optical variability associated with changes in accretion state. We investigate the optical variability of turn-on CL AGNs, selected through the emergence or strengthening of broad emission lines, using g-band light curves from the Zwicky Transient Facility. For a final sample of 106 objects, we measure structure-function amplitudes at three fixed rest-frame timescales, SF30, SF150 and SF300, and examine their dependence on optical luminosity, Eddington ratio, black-hole mass and rest-frame wavelength using Spearman-rank correlations and multivariate regressions. The strongest trends are found on monthly timescales. SF30 is significantly anti-correlated with both optical luminosity and Eddington ratio, with Spearman coefficients of ρ = −0.39 and ρ = −0.33, respectively. These negative trends remain in multivariate regressions after accounting for black-hole mass and rest-frame wavelength. The dependence weakens at longer timescales: SF150 shows only weak evidence for luminosity or Eddington-ratio dependence, while SF300 shows no robust dependence. Black-hole mass shows no robust single-parameter correlation, and its multivariate coefficients depend on the model form, indicating that its apparent role is affected by covariance with luminosity and Eddington ratio. The rest-frame wavelength term is generally negative, but cannot be separated from redshift in this single-band analysis. These results suggest that monthly optical variability in turn-on CL AGNs is more closely linked to bright-state accretion properties than variability on half-year to year-long timescales.