ACCRETION DISK MODELS FOR OPTICAL AND ULTRAVIOLET MICROVARIABILITY IN ACTIVE GALACTIC NUCLEI

We describe scenarios for rapid, low-amplitude UV and optical variability in AGNs through phenomenologically modeled asymmetries on accretion disks, which can represent perturbations arising from magnetic, gravitational, and other instabilities. The simulations were done for both the more familiar alpha-disks, where the viscous shear, t(rphi), is proportional to the total pressure, as well as for the beta-disk geometry, where the shear is proportional to only the gas pressure; a range of parameters for flare properties and disk structures were considered. Our calculations have taken into account geometric effects of eclipsing and time delays, gravitational and rotation-induced frequency shifts and flux enhancements, as well as assumed intrinsic fluctuations of these spots. Results show the power density spectra have slopes between -1.4 and -2.1, similar to the observed range. While these phenomenological models are promising, each full-fledged model will have to include the detailed physics for a specific instability mechanism.