How Long Will the Quasar UV/Optical Flickering Be Damped?

Abstract

The UV/optical light curves of Active Galactic Nuclei (AGNs) are commonly described by the Damped Random Walk (DRW) model. However, the physical interpretation of the damping timescale, a key parameter in the DRW model, remains unclear. Particularly, recent observations indicate a weak dependence of the damping timescale upon both wavelength and accretion rate, clearly being inconsistent with the accretion-disk theory. In this study, we investigate the damping timescale in the framework of the Corona Heated Accretion disk Reprocessing (CHAR) model, a physical model that describes AGN variability. We find that while the CHAR model can reproduce the observed power spectral densities of the 20 yr light curves for 190 sources from Stone et al., the observed damping timescale, as well as its weak dependence on wavelength, can also be well recovered through fitting the mock light curves with DRW. We further demonstrate that such weak dependence is artificial due to the effect of inadequate durations of light curves, which leads to best-fitting damping timescales lower than the intrinsic ones. After eliminating this effect, the CHAR model indeed yields a strong dependence of the intrinsic damping timescale on the bolometric luminosity and rest-frame wavelength. Our results highlight the demand for sufficiently long light curves in AGN variability studies and important applications of the CHAR model in such studies.

A New Timescale–Mass Scaling for the Optical Variation of Active Galactic Nuclei across the Intermediate-mass to Supermassive Scales

“…Interestingly, our mass index agrees well with that derived by Arévalo et al (2024) using a different method. Furthermore, our mass index is quite consistent with the prediction of the simple static standard thin disk, i.e., t µ M (Zhou et al 2024), predicted by the corona heated accretion disk reprocessing model (Sun et al 2020a(Sun et al , 2020b, which takes the time-dependent evolution of the standard thin disk into account.…”

“…1.9 days. The 1σ upper limit for the damping timescale is already 3.3 days, suggesting that a single 1 month long light curve is far from any firm conclusion on the true intrinsic damping timescale for NGC 4395 (e.g., Hu et al 2024;Zhou et al 2024). According to Hu et al (2024, see their Figure 8), if the true damping timescale is 1.4 days for NGC 4395, the retrieved damping timescale has ∼40% (1σ confidence level) dispersion given a single light curve with a 1 month long baseline.…”

Sizes of Active Galactic Nucleus Inhomogeneous Disks—Large in Microlensing, Small in Reverberation Mapping

“…Dexter & Agol (2011) assume that the damping timescale is ∼200 days, corresponding to the observed damping timescales in quasar optical light curves (MacLeod et al 2010). There is increasing evidence that the 200 days damping timescale is strongly underestimated (e.g., Kozłowski 2017;Suberlak et al 2021;Zhou et al 2024). Cai et al (2016) proposed that the timescale-dependent color variations in quasars can be reproduced if the damping timescale anticorrelates with radius.…”