A characteristic optical variability time scale in astrophysical accretion disks

Abstract

Accretion disks around supermassive black holes in active galactic nuclei produce continuum radiation at ultraviolet and optical wavelengths. Physical processes in the accretion flow lead to stochastic variability of this emission on a wide range of time scales. We measured the optical continuum variability observed in 67 active galactic nuclei and the characteristic time scale at which the variability power spectrum flattens. We found a correlation between this time scale and the black hole mass extending over the entire mass range of supermassive black holes. This time scale is consistent with the expected thermal time scale at the ultraviolet-emitting radius in standard accretion disk theory. Accreting white dwarfs lie close to this correlation, suggesting a common process for all accretion disks.

Optical Variability of Quasars with 20-Year Photometric Light Curves

“…Kozłowski (2017) emphasized the importance of the length of the light curve in constraining the DRW damping timescale. The bestfit 𝜏 DRW could be significantly underestimated if the light curve is not long enough, as independently confirmed in other studies with simulated light curves (e.g., Suberlak et al 2021;Burke et al 2021). The fact that the average 𝜏 DRW continues to rise as the baseline increases indicates that even the 20-year baseline is probably not long enough to well constrain 𝜏 DRW in some S82 quasars.…”

“…They found a strong positive correlation between 𝜏 DRW and black hole mass, which extends to the stellar mass regime with optical variability measured for nova-like accreting white dwarfs (Scaringi et al 2015). Compared with higher-order Gaussian process models, the DRW model contains a single characteristic timescale, making it easier to interpret the variability and to connect variability to the underlying physical processes (e.g., Burke et al 2021;Sun et al 2020).…”

“…We follow the standard practice in the literature to fit a DRW model to the quasar light curve (e.g., Kelly et al 2009;Kozłowski et al 2010;MacLeod et al 2010;Suberlak et al 2021;Burke et al 2021). The details of the DRW modeling are provided in Appendix A2.…”

“…Earlier studies of quasar variability already hinted at such a characteristic variability timescale and its possible dependence on the physical properties of quasars such as the black hole mass (e.g., Collier & Peterson 2001;Kelly et al 2009), but the exact form of the dependence is debated (e.g., MacLeod et al 2010;Simm et al 2016). Recently, Burke et al (2021) measured the damping timescales using the DRW model for a sample of Active Galactic Nuclei (AGNs) with high-quality optical light curves over a large dynamic range in black hole mass. They found a strong positive correlation between 𝜏 DRW and black hole mass, which extends to the stellar mass regime with optical variability measured for nova-like accreting white dwarfs (Scaringi et al 2015).…”

Using AGN lightcurves to map accretion disc temperature fluctuations

“…For example, Dexter & Agol (2011) argue that disc variability is largely due to inhomogeneous temperature fluctuations. Others argue for intrinsic thermal fluctuations in the disc as the origin of the variability, based on the fact that the observed variability time-scales are typical of the thermal time-scales at the disc radii producing the observed flux (e.g., Kelly et al 2009;Burke et al 2021). These long time-scales are a significant problem for the lamppost model, as they are much longer than any characteristic time-scale associated with the very inner regions of the disc.…”