The Timescale-Dependent Color Variability of Quasars Viewed with GALEX

Abstract

In recent work done by Sun et. al., the color variation of quasars, namely the bluer-when-brighter trend, was found to be timescale-dependent using SDSS $g/r$ band light curves in the Stripe 82. Such timescale dependence, i.e., bluer variation at shorter timescales, supports the thermal fluctuation origin of the UV/optical variation in quasars, and can be well modeled with the inhomogeneous accretion disk model. In this paper, we extend the study to much shorter wavelengths in the rest frame (down to extreme UV), using GALaxy Evolution eXplorer (GALEX) photometric data of quasars collected in two ultraviolet bands (near-UV and far-UV). We develop Monte-Carlo simulations to correct possible biases due to the considerably larger photometric uncertainties in GALEX light curves (particularly in far-UV, comparing with SDSS $g/r$ bands), which otherwise could produce artificial results. We securely confirm the previously discovered timescale dependence of the color variability with independent datasets and at shorter wavelengths. We further find the slope of the correlation between the amplitude of color variation and timescale however appears even steeper than that predicted by the inhomogeneous disk model, which assumes that disk fluctuations follow damped random walk process. In line with the much flatter structure function observed in far-UV comparing with that at longer wavelengths, this implies deviation from DRW process in the inner disk where rest frame extreme UV radiation is produced.

8 pages, 6 figures. Accepted by APJ

References in corpus (11)

• A Catalog of Quasar Properties from SDSS DR7
• Modeling the Time Variability of SDSS Stripe 82 Quasars as a Damped Random Walk
• Is Quasar Optical Variability a Damped Random Walk?
• Space Telescope and Optical Reverberation Mapping Project. II. Swift and HST Reverberation Mapping of the Accretion Disk of NGC 5548
• Quasar Accretion Disks Are Strongly Inhomogeneous
• Space Telescope and Optical Reverberation Mapping Project. I. Ultraviolet Observations of the Seyfert 1 Galaxy NGC 5548 with the Cosmic Origins Spectrograph on Hubble Space Telescope
• Swift monitoring of NGC 5548: X-ray reprocessing and short term UV/optical variability
• Anatomy of the AGN in NGC 5548: II. The Spatial, Temporal and Physical Nature of the Outflow from HST/COS Observations
• An accretion disc model for quasar optical variability
• The Discovery of Timescale-Dependent Color Variability of Quasars
• The Second Galex Ultraviolet Variability (GUVV-2) Catalog

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• A new timescale-mass scaling for the optical variation of active galactic nuclei across the intermediate-mass to supermassive scales
• The Relation between X-ray and Ultraviolet Variability of Quasars
• Extreme Variability Quasars in Their Various States. II: Spectral Variation Revealed with Multi-epoch Spectra