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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. 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 quasars with light curves (LCs) of almost nightly cadence from > 5 years of observations by the NASA/ATLAS project, which provides 2 billion magnitude pairs for a structure function (SF) analysis. When time is expressed in units of orbital or thermal timescale 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 as a main cause. Over a > 1 dex range in time, the fractional variability amplitude follows log(A/A0) 1/2 x log(dt/tth). 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 over-luminous when viewed pole-on and under-luminous when viewed at high inclination. In radio-quiet quasars, the viewing angle is usually unknown, although spectroscopic indicators have been proposed. We use the universality in the variability SF of quasar LCs to infer offsets between observed and intrinsic luminosity due to the anisotropy. We select two subsamples from the above for this work, one is a sample of 183 quasars with measured H-beta lines as well as another sample of 753 quasars with CIV and MgII lines. Starting from the proposed orientation indicators, we expect quasars with narrower H-beta lines and with more blueshifted CIV lines to be viewed more pole-on and thus appear over-luminous. In contrast, our SF analysis finds that presumed pole-on discs appear under-luminous, consistently for both line indicators. The simplest explanation would be that quasars with highly blueshifted CIV lines have dusty outflows, which render the accretion disc under-luminous, irrespective of inclination angle. In our SF analysis, we corroborate a known suppression at timescale shorter than the RW regime, which is caused by an unknown reason. We propose that this is due to a "smearing" effect caused by the synchronized emission from different parts of the disc reaching us after different time lags. We seperate the sample of ~5000 quasars by their redshifts and luminosities, fit the SFs of every groups to find the dt breaks from the RW regime, then investigate the relation between those breaks and the rest-frame wavelengths and luminosities. If our assumption is correct, we will see a relation following the disc size predicted by the thin-disc models. This will be an innovative method measuring the disc size other than previous methods such as micro-lensing variability and continuum reverberation mapping. |
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