Quasar Variability and its Relation to Accretion Disc Properties
Creators: name: Tang, Ji-Jia
Publisher: name: The Australian National University
Year: 2024

description: Rapidly growing black holes are surrounded by accretion discs that make quasars the brightest objects in the Universe. Their brightness is known to be variable, but the causes of this are not implied by simple disc models and still debated. Due to the small size of accretion discs and their great distance, there are no resolved images addressing the puzzle. We study the dependence of their variability on luminosity, wavelength and orbital/thermal timescale (torb, or tth). We use over 5,000 of the most luminous quasars with light curves (LCs) of almost nightly cadence from &gt; 5 years of observations by the NASA/ATLAS project, which provides about 2 billion magnitude pairs for a structure function (SF) analysis. When time is expressed in units of torb or tth in thin-disc models in the random walk (RW) regime, we find a universal structure function, independent of luminosity and wavelength, supporting the model of magneto-rotational instabilities (MRIs) as a main cause. In other words, we find evidence showing the variability is caused by the turbulence in the disc. Over a &gt; 1 dex range in time, the fractional variability amplitude, A, follows log(A/A0) = 1/2 x log(dt/tth), where A0 is a constant. Deviations from the universality in the RW regime may hold clues on the orientation of discs. Given the anisotropic emission from quasar accretion discs, their viewing angle affects estimates of the quasar luminosity, black hole mass, and Eddington ratio. Discs appear overluminous when viewed pole-on and underluminous when viewed at high inclination. In radio-quiet quasars, the viewing angle is usually unknown, although spectroscopic indicators have been proposed. The existence of the universal variability SF of quasar suggests that any offset from the mean relation can be caused by incorrect torb and thus incorrect luminosities. We correlate these offsets with suggested inclination indicators of spectral lines, using a sample of 183 luminous quasars with measured H-beta lines as well as 753 quasars with C IV and Mg II lines. Starting from the proposed orientation indicators, we expect quasars with narrower H-beta lines and with more blueshifted C IV lines to be viewed more pole-on and thus appear overluminous. In contrast, our SF analysis finds that presumed pole-on discs appear underluminous, consistently for both line indicators. We discuss possible explanations for the behaviour of quasars with highly blueshifted C IV lines irrespective of inclination angle, including dusty outflows that might render the accretion disc underluminous and flatter disc temperature profiles with longer torb than in thin-disc models but reach no satisfying conclusion. The stochastic photometric variability of quasars is known to follow a random-walk phenomenology on emission timescales of weeks to months. Some high- cadence restframe optical monitoring of the past has hinted at a suppression of variability amplitudes on shorter timescales of a few days, opening the question of what drives the suppression and how it might scale with quasar properties. Here, we study a few thousand of the highest-luminosity quasars in the sky, mostly in the luminosity range of Lbol= [46.4, 47.3] and redshift range of z = [0.7, 2.4]. We use a dataset from the NASA/ATLAS facility with nightly cadence, weather permitting, which has been used before to quantify strong regularity in longer-term restframe- UV variability. As we focus on a careful treatment of short timescales across the sample, we find no evidence of any systematic breaks or suppression in the UV variability structure function at short timescales; instead, the data is consistent with a single-slope random walk across restframe timescales of dt = [10, 250] days. We also perform a numerical calculation result of the accretion disc size and tth of quasars using the definition of flux-weighted size, providing useful information for future studies.