The color variability of quasars

Kasper B. Schmidt, Hans Walter Rix, Joseph C. Shields, Matthias Knecht, David W. Hogg, Dan Maoz, Jo Bovy

    Research output: Contribution to journalArticlepeer-review


    We quantify quasar color variability using an unprecedented variability database-ugriz photometry of 9093 quasars from Sloan Digital Sky Survey (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 disk.

    Original languageEnglish (US)
    Article number147
    JournalAstrophysical Journal
    Issue number2
    StatePublished - Jan 10 2012


    • accretion
    • accretion disks
    • galaxies: active
    • galaxies: nuclei
    • quasars: emission lines
    • quasars: general

    ASJC Scopus subject areas

    • Astronomy and Astrophysics
    • Space and Planetary Science


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