The Color Variability of Quasars

TL;DR

This study analyzes quasar color variability using extensive SDSS data, revealing that after correction for emission line effects, quasar color changes are uniform across redshift and luminosity, driven by physical mechanisms like accretion disc flares.

Contribution

It provides the first large-scale quantification of quasar color variability, correcting for emission line effects, and links variability characteristics to physical processes in accretion discs.

Findings

Quasars become bluer when they brighten.

Color variability is independent of redshift, luminosity, and black hole mass after correction.

Fast, low-amplitude brightness variations lead to more color variability.

Abstract

We quantify quasar color-variability using an unprecedented variability database - ugriz photometry of 9093 quasars from SDSS Stripe 82, observed over 8 years at ~60 epochs each. We confirm previous reports that quasars become bluer when brightening. We find a redshift dependence of this blueing in a given set of bands (e.g. g and r), but show that it is the result of the flux contribution from less-variable or delayed emission lines in the different SDSS bands at different redshifts. After correcting for this effect, quasar color-variability is remarkably uniform, and independent not only of redshift, but also of quasar luminosity and black hole mass. The color variations of individual quasars, as they vary in brightness on year timescales, are much more pronounced than the ranges in color seen in samples of quasars across many orders of magnitude in luminosity. This indicates distinct physical mechanisms behind quasar variability and the observed range of quasar luminosities at a given black hole mass - quasar variations cannot be explained by changes in the mean accretion rate. We do find some dependence of the color variability on the characteristics of the flux variations themselves, with fast, low-amplitude, brightness variations producing more color variability. The observed behavior could arise if quasar variability results from flares or ephemeral hot spots in an accretion disc.