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

TL;DR

This study reveals a universal variability pattern in luminous quasars' light curves, supporting magneto-rotational instabilities as a key driver, and shows that variability scales with thermal timescales across different luminosities and wavelengths.

Contribution

It demonstrates a universal structure function in quasar variability when scaled by thermal timescales, indicating a common underlying physical process.

Findings

Universal structure function independent of luminosity and wavelength

Variability amplitude scales with thermal timescale as log(A/A0) ≈ 0.5 × log(Δt/t_th)

Supports magneto-rotational instabilities as main cause of variability

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 $\log (A/A_0) \simeq 1/2 \times \log (\Delta t/t_{\rm th})$. Deviations from the universality may hold clues on the structure and orientation of disks.