Recent observations of extreme variability in Active Galactic Nuclei have pushed standard viscous accretion disc models over an edge. "Extreme reprocessing" where an erratically variable central quasi-point source is entirely responsible for heating an otherwise cold and passive low-viscosity disc, may be the best route forward.

For consistency with Nature policy, the version posted here is the text as submitted to Nature Astronomy in November 2017. The final version is available at the DOI below, and differs slightly in wording. The final version can be posted in six months