Are the variations in quasar optical flux driven by thermal fluctuations?

Brandon C. Kelly, Jill Bechtold, Aneta Siemiginowska

Research output: Contribution to journalArticlepeer-review

585 Scopus citations


We analyze a sample of optical light curves for 100 quasars, 70 of which have black hole mass estimates. Our sample is the largest and broadest used yet for modeling quasar variability. The sources in our sample have z < 2.8, 1042 ≲ λL λ(5100 ) ≲ 10 46, and 106 ≲ M BH/M ≲ 1010. We model the light curves as a continuous time stochastic process, providing a natural means of estimating the characteristic timescale and amplitude of quasar variations. We employ a Bayesian approach to estimate the characteristic timescale and amplitude of flux variations; our approach is not affected by biases introduced from discrete sampling effects. We find that the characteristic timescales strongly correlate with black hole mass and luminosity, and are consistent with disk orbital or thermal timescales. In addition, the amplitude of short-timescale variations is significantly anticorrelated with black hole mass and luminosity. We interpret the optical flux fluctuations as resulting from thermal fluctuations that are driven by an underlying stochastic process, such as a turbulent magnetic field. In addition, the intranight variations in optical flux implied by our empirical model are ≲0.02 mag, consistent with current microvariability observations of radio-quiet quasars. Our stochastic model is therefore able to unify both long- and short-timescale optical variations in radio-quiet quasars as resulting from the same underlying process, while radio-loud quasars have an additional variability component that operates on timescales ≲1 day.

Original languageEnglish (US)
Pages (from-to)895-910
Number of pages16
JournalAstrophysical Journal
Issue number1
StatePublished - 2009


  • Accretion, accretion disks
  • Galaxies: active
  • Methods: data analysis
  • Quasars: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


Dive into the research topics of 'Are the variations in quasar optical flux driven by thermal fluctuations?'. Together they form a unique fingerprint.

Cite this