Inhomogeneous Quasar Accretion Disks

Standard thin disk accretion has been used for nearly 40 years to explain the visible light from quasars. However, it has trouble explaining the broad spectra seen in quasars, as well as the simultaneous observed variability in different optical wavebands. Further, microlensing observations find that quasar accretion disks are larger than expected by factors of a few. Long term monitoring shows that quasars vary stochastically by 10-20% in the optical. It's unlikely that the disk all varies coherently, so the fluctuations in small patches of the disk may be much larger. If the average properties of the disk agree with standard thin accretion disk theory, but disks are highly inhomogeneous with factors of a few in temperature present at all radii, then many puzzling observations can be explained. An inhomogeneous disk will appear larger at a given wavelength, since the range in temperatures means that higher temperatures will be present at larger radius than in a thin disk model. The presence of a range of temperatures in a small portion of the disk naturally produces simultaneous variability at many different wavebands, while the overall spectrum from an inhomogeneous disk will be broader than a thin disk. It turns out that the degree of inhomogeneity required to explain the disk sizes produces spectra that agree well with observed quasar spectra. We demonstrate this by producing toy models of inhomogeneous disks and computing their size vs. wavelength from microlensing simulations and their optical spectra.

[Sample temperature maps] [Size vs. Wavelength] [Optical spectra] [Combined constraints]

References: Dexter, J. & Agol, E., ApJL, 727, 24.