Quasar Accretion Disks Are Strongly Inhomogeneous

Abstract

Active galactic nuclei (AGN) have been observed to vary stochastically with 10-20 rms amplitudes over a range of optical wavelengths where the emission arises in an accretion disk. Since the accretion disk is unlikely to vary coherently, local fluctuations may be significantly larger than the global rms variability. We investigate toy models of quasar accretion disks consisting of a number of regions, n, whose temperatures vary independently with an amplitude of σ_T in dex. Models with large fluctuations (σ_T=0.35-0.50) in 100-1000 independently fluctuating zones for every factor of two in radius can explain the observed discrepancy between thin accretion disk sizes inferred from microlensing events and optical luminosity while matching the observed optical variability. For the same range of σ_T, inhomogeneous disk spectra provide excellent fits to the HST quasar composite without invoking global Compton scattering atmospheres to explain the high levels of observed UV emission. Simulated microlensing light curves for the Einstein cross from our time-varying toy models are well fit using a time-steady power-law temperature disk, and produce magnification light curves that are consistent with current microlensing observations. Deviations due to the inhomogeneous, time-dependent disk structure should occur above the 1% level in the light curves, detectable in future microlensing observations with millimag sensitivity.

6 pages, 5 figures, accepted by ApJL

References in corpus (9)

• Stability of Relativistic Jets from Rotating, Accreting Black Holes via Fully Three-Dimensional Magnetohydrodynamic Simulations
• Global General Relativistic MHD Simulation of a Tilted Black-Hole Accretion Disk
• Three-Dimensional Simulations of Magnetized Thin Accretion Disks around Black Holes: Stress in the Plunging Region
• Accretion Discs with Strong Toroidal Magnetic Fields
• The Spatial Structure of An Accretion Disk
• Microlensing variability in the gravitationally lensed quasar QSO 2237+0305 = the Einstein Cross. II. Energy profile of the accretion disk
• The UV Continuum of Quasars: Models and SDSS Spectral Slopes
• The multiple quasar Q2237+0305 under a microlensing caustic
• Photon Bubbles and the Vertical Structure of Accretion Disks