  Rapid, large amplitude variability at optical to X-ray wavelengths is now
seen in an increasing number of Seyfert galaxies and luminous quasars. The
variations imply a global change in accretion power, but are too rapid to be
communicated by inflow through a standard thin accretion disc. Such discs are
long known to have difficulty explaining the observed optical/UV emission from
active galactic nuclei. Here we show that alternative models developed to
explain these observations have larger scale heights and shorter inflow times.
Accretion discs supported by magnetic pressure in particular are geometrically
thick at all luminosities, with inflow times as short as the observed few year
timescales in extreme variability events to date. Future time-resolved,
multi-wavelength observations can distinguish between inflow through a
geometrically thick disc as proposed here, and alternative scenarios of extreme
reprocessing of a central source or instability-driven limit cycles.
