The Future of X-ray Reverberation from AGN
  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. 
A.C. Fabian, W.N. Alston, E.M. Cackett, E.Kara, P.Uttley, D.R.Wilkins
astro-ph.HE