The Discovery of Timescale-Dependent Color Variability of Quasars

Abstract

Quasars are variable on timescales from days to years in UV/optical, and generally appear bluer while they brighten. The physics behind the variations in fluxes and colors remains unclear. Using SDSS g and r band photometric monitoring data of quasars in Stripe 82, we find that although the flux variation amplitude increases with timescale, the color variability exhibits opposite behavior. The color variability of quasars is prominent at timescales as short as ~ 10 days, but gradually reduces toward timescales up to years. In other words, the variable emission at shorter timescales is bluer than that at longer timescales. This timescale dependence is clearly and consistently detected at all redshifts from z = 0 to 3.5, thus can not be due to contaminations to broadband photometry from emission lines which do not respond to fast continuum variations. The discovery directly rules out the possibility that simply attributes the color variability to contamination from a non-variable redder component, such as the host galaxy. It can not be interpreted as changes in global accretion rate either. The thermal accretion disk fluctuation model is favored, in the sense that fluctuations in the inner hotter region of the disk are responsible for short term variations, while longer term and stronger variations are expected from larger and cooler disk region. An interesting implication is that one can use quasar variations at different timescales to probe disk emission at different radii.

Accepted for publication in ApJ, 22 pages, 5 figures

References in corpus (6)

• Modeling the Time Variability of SDSS Stripe 82 Quasars as a Damped Random Walk
• Is Quasar Optical Variability a Damped Random Walk?
• Quasar Accretion Disks Are Strongly Inhomogeneous
• The UV Continuum of Quasars: Models and SDSS Spectral Slopes
• An accretion disc model for quasar optical variability
• Size and disk-like shape of the broad-line region of ESO399-IG20

Cited by in corpus (9)

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• Appearance vs Disappearance of broad absorption line troughs in quasars
• An Accretion Disc-Irradiation Hybrid Model for The Optical/UV Variability in Radio-Quiet Quasars
• The Relation between X-ray and Ultraviolet Variability of Quasars
• Chromatic Microlensing Time Delays
• The Host Galaxy Fluxes of Active Galaxy Nuclei Are Generally Overestimated by the Flux Variation Gradient Method