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2020-10-26 Abstract

Title: The Variability Structure Function of the Most Luminous Quasars
 
Speaker:  Ji-Jia Tang (ANU)
 
Date: October 26 at 12:20
 
Location: R501, General Building II
 
Abstract: 
We study the variability of the brightest known quasars in the sky by selecting ∼1200 objects within the ∼ 3π srad footprint of the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey. Our quasar sample covers a redshift range from z = 1 to z ≈ 4 and has a median redshift of z = 1.53. The large sky coverage of ATLAS provides data for quasars of bolometric luminosity up to log Lbol/(erg s−1) ≈ 48.6. The ATLAS data include light curves in two filters, the cyan (c) and orange (o) bands, over four years duration, allowing us to probe rest-frame time-scales from less than a day to ∼1 year. We find that the amplitude of quasar variability is a strong function of the time interval ∆t and the rest-frame wavelength λrf and a weaker function of luminosity Lbol. At all λrf and Lbol and ∆t > 3 months the amplitude increases with A ∝ ∆t0.478±0.014, which is suggestive of an underlying variability process with a power spectral density of P( f ) ∝ f −2. At constant variability amplitude, our best fits reveal a relationship close to ∆t ∝ Lbol1/2λrf2 , which is the functional form predicted for thermal and orbital time-scales by the thin-disc theory. Combined with recent literature work, this behaviour is observed over more than two orders of magnitude in luminosity. 
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