  XMM-Newton is capable of making a transformational advance in our
understanding of how luminous accreting black holes work, by dedicating about
10 per cent of future observing time to long observations, of order
Megaseconds, to X-ray variable Active Galactic Nuclei (AGN) research. This
would enable reverberation studies, already a commonplace feature of AGN, to
proceed to the next level and follow the behaviour of the powerful dynamic
corona. Such a dedicated legacy programme can only be carried out with
XMM-Newton.
