A. Zoghbi, D. R. Wilkins, L. Brenneman, G. Miniutti, G. Matt, J. Garcia, E. Kara, E. Cackett, B. De Marco, M. Dovciak, 2019. Whitepaper submitted to the 2020 Decadal Survey
X-ray reverberation mapping has emerged as a powerful probe of microparsec scales around AGN, and with high sensitivity detectors, its full potential in echo-mapping the otherwise inaccessible disk-corona at the black hole horizon scale will be revealed.