The Timescale-Dependent Color Variability of Quasars Viewed with GALEX

TL;DR

This study confirms that quasar color variability depends on timescale across UV wavelengths, supporting thermal fluctuation models but also indicating deviations in the inner disk behavior from standard assumptions.

Contribution

It extends previous work by analyzing shorter UV wavelengths with GALEX data and develops Monte-Carlo simulations to correct for photometric uncertainties.

Findings

Confirmed timescale-dependent color variability in UV bands

Found steeper correlation between color variation amplitude and timescale

Indicated deviations from damped random walk process in inner accretion disk

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.