Are the Variations in Quasar Optical Flux Driven by Thermal Fluctuations?

TL;DR

This study models quasar optical flux variability using a stochastic process, revealing correlations with black hole properties and supporting thermal fluctuations as a key driver of observed variations.

Contribution

It introduces a Bayesian stochastic modeling approach that accurately estimates quasar variability time scales and amplitudes, linking them to physical processes.

Findings

Characteristic time scales correlate with black hole mass and luminosity.

Amplitude of short-term variations anti-correlates with black hole mass.

Optical flux fluctuations are consistent with thermal fluctuations driven by stochastic processes.

Abstract

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 and 10^6 < M_BH < 10^10. We model the light curves as a continuous time stochastic process, providing a natural means of estimating the characteristic time scale and amplitude of quasar variations. We employ a Bayesian approach to estimate the characteristic time scale and amplitude of flux variations; our approach is not affected by biases introduced from discrete sampling effects. We find that the characteristic time scales strongly correlate with black hole mass and luminosity, and are consistent with disk orbital or thermal time scales. In addition, the amplitude of short time scale variations is significantly anti-correlated 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 time scale 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 time scales < 1 day.