Corona-heated Accretion-disk Reprocessing: A Physical Model to Decipher the Melody of AGN UV/Optical Twinkling

Abstract

Active galactic nuclei (AGNs) have long been observed to "twinkle" (i.e., their brightness varies with time) on timescales from days to years in the UV/optical bands. Such AGN UV/optical variability is essential for probing the physics of supermassive black holes (SMBHs), the accretion disk, and the broad-line region. Here we show that the temperature fluctuations of an AGN accretion disk, which is magnetically coupled with the corona, can account for observed high-quality AGN optical light curves. We calculate the temperature fluctuations by considering the gas physics of the accreted matter near the SMBH. We find that the resulting simulated AGN UV/optical light curves share the same statistical properties as the observed ones as long as the dimensionless viscosity parameter α, which is widely believed to be controlled by magnetohydrodynamic (MHD) turbulence in the accretion disk, is about 0.01-0.2. Moreover, our model can simultaneously explain the larger-thanexpected accretion disk sizes and the dependence of UV/optical variability upon wavelength for NGC 5548. Our model also has the potential to explain some other observational facts of AGN UV/optical variability, including the timescale-dependent bluer-when-brighter color variability and the dependence of UV/optical variability on AGN luminosity and black hole mass. Our results also demonstrate a promising way to infer the black-hole mass, the accretion rate, and the radiative efficiency, thereby facilitating understanding of the gas physics and MHD turbulence near the SMBH and its cosmic mass growth history by fitting the AGN UV/optical light curves in the era of time-domain astronomy.

Optical Variability of Quasars with 20-Year Photometric Light Curves

“…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).…”

“…One possibility, as suggested by Burke et al (2021), is that the observed UV/optical variability is driven by processes in the inner (UV-emitting) part of the accretion disk, which rapidly propagates outwards at the Alfvén speed, during which the characteristic variability timescale is more or less preserved. Alternatively, the observed damping timescale may be the thermal timescale averaged over different radii, leading to a shallower wavelength dependence (e.g., Sun et al 2020). Further development To obtain this DRW prediction, we sample 500 predicted DRW structure functions from each target in the ensemble, drawn from a Gaussian distribution using its best-fit DRW parameters and their uncertainties.…”

Tracking the year-to-year variation in the spectral energy distribution of the narrow-line Seyfert 1 galaxy Mrk 335

“…Despite the different timescales involved, it might be possible that the abrupt change in the X-ray flux and the more gradual change in the optical emission regions of the source are possibly connected through some process. For instance, Sun et al (2020) suggest a model which show that the magnetic coupling between the compact corona and the outer cold accretion disc might exist. However, based on the pre-2007 X-ray light curve (e.g.…”

AGN STORM 2. I. First results: A Change in the Weather of Mrk 817

“…Despite the strong and variable absorption observed in the X-rays and the UV absorption lines and the lack of an X-ray/ UV correlation, the and optical continuum variability and continuum lags look very similar to what would be expected. Either the disk sees a different source of irradiating photons than the X-rays we observe (e.g., the X-ray absorber is not located between the X-ray and UV/optical continuum region or does not block all lines of sight between those regions), or a different mechanism drives variability in the disk (e.g., coronaheated accretion disk reprocessing; Sun et al 2020aSun et al , 2020b.…”