Universality in the Random Walk Structure Function of Luminous Quasi-Stellar Objects

Abstract

Rapidly growing black holes are surrounded by accretion disks that make them the brightest objects in the Universe. Their brightness is known to be variable, but the causes of this are not implied by simple disk models and still debated. Due to the small size of accretion disks and their great distance, there are no resolved images addressing the puzzle. In this work, we study the dependence of their variability on luminosity, wavelength and orbital/thermal timescale. We use over 5,000 of the most luminous such objects with light curves of almost nightly cadence from $>5$ years of observations by the NASA/ATLAS project, which provides 2 billion magnitude pairs for a structure function analysis. When time is expressed in units of orbital or thermal time scale in thin-disk models, we find a universal structure function, independent of luminosity and wavelength, supporting the model of magneto-rotational instabilities as a main cause. Over a $>1$~dex range in time, the fractional variability amplitude follows $\log (A/A_0)\simeq 1/2\times \log (\Delta t/t_{\mathrm{th}})$. Deviations from the universality may hold clues on the structure and orientation of disks.

21 pages, 5 figures

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Cited by in corpus (9)

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• A new timescale-mass scaling for the optical variation of active galactic nuclei across the intermediate-mass to supermassive scales
• Examining AGN UV/Optical Variability Beyond the Simple Damped Random Walk. II. Insights from 22 Years Observations of SDSS, PS1 and ZTF
• Temperature Profiles of Accretion Disks in Luminous Active Galactic Nuclei derived from Ultraviolet Spectroscopic Variability
• Unlocking AGN Variability with Custom ZTF Photometry for High-Fidelity Light Curves and Robust Selection
• The ensemble broad-frequency power spectrum of Stripe-82 quasars from multiple surveys
• Optical Variability Structure Function of Low-Luminosity AGN using ATLAS Lightcurves
• Probing Quasar Viewing Angle with the Variability Structure Function