Optical variability and optical--mid-infrared dust lags in Type~1 changing-look AGNs

Abstract

Changing-look active galactic nuclei (CL AGNs) show large changes in luminosity and optical spectral state on time-scales of a few years, and provide a valuable probe of time-dependent accretion in the disc-BLR-torus system. We present a systematic statistical study of their optical variability in a well-defined Type-1 phase, using g- and r-band light curves from the Zwicky Transient Facility for 165 CL AGNs. A subsample of 34 objects also has NEOWISE W1 and W2 light curves, which we use to measure optical-mid-infrared time lags. We use structure functions and a damped random-walk model to characterize variability amplitudes and time-scales on rest-frame scales from tens to a few hundred days, and examine their dependence on black hole mass, luminosity, and Eddington ratio. In the Type-1 phase, the short-time-scale optical variability amplitude on about 30-day time-scales shows little dependence on black hole mass, luminosity, or Eddington ratio. By contrast, the longer-term amplitudes on 150-300 day time-scales, as well as the damped random-walk time-scales, increase slowly with black hole mass and luminosity, but still show no clear dependence on Eddington ratio. The sample shows a ubiquitous bluer-when-brighter trend and larger variability at shorter wavelengths, consistent with continuum variability from a multi-temperature accretion disc. For the NEOWISE subsample, the dust lag-luminosity relation inferred from the optical-mid-infrared lags is similar to that of normal Type-1 AGNs. Overall, CL AGNs in the Type-1 phase behave like normal Type-1 AGNs within the standard disc-BLR-dusty torus framework, but are more prone to large continuum reconfigurations on year-like time-scales.

Figures (9)

• Figure 1: Top: distribution of the 165 Type-1 CL AGNs in the redshift–luminosity plane. Bottom: redshift distributions of the full optical sample and the 34-object MIR subsample.
• Figure 2: Correlations between optical variability amplitudes on rest-frame time-scales of 30, 150 and 300 d and black hole mass $M_{\rm BH}$ for the 165 Type-1 CL AGNs. From left to right: $SF_{30}$, $SF_{150}$ and $SF_{300}$ as a function of $M_{\rm BH}$.
• Figure 3: Same as Fig. \ref{['fig:sf_mbh']}, but as a function of bolometric luminosity $L_{\rm bol}$.
• Figure 4: Correlations between the asymptotic variability amplitude $SF_{\infty}$ and (from left to right) black hole mass $M_{\rm BH}$, bolometric luminosity $L_{\rm bol}$, and Eddington ratio $\lambda_{\rm Edd}$ for the 165 Type-1 CL AGNs.
• Figure 5: Distribution of the colour--brightness slope $\beta$ for the 165 Type-1 CL AGNs, where $\beta>0$ corresponds to a bluer-when-brighter trend.