The multimessenger view of coalescing massive black hole binaries.

LISA will discover massive black hole binary coalescence, among the loudest sources of gravitational waves in the universe. If their coalescence occurs in a gas-rich environment, light from the surrounding plasma accompanies the emission of gravitational waves. Light and gravity waves carry exquisite information on the masses and spins of the two black holes, on the magneto-hydrodynamical behaviour of fluid flows in the violently changing spacetimes around the in spiralling black holes, and on jet launching around newly formed horizons. This will enable exquisite tests on the expansion rate of the universe, and on the propagation properties of gravitational wave across cosmic distances. The possibility of performing such tests depends critically on LISA’s capability to localise massive black hole coalescences and on the ability of telescopes, and in particular the X-ray observatory Athena, to slew across the sky as fast as possible, following an alert, to search for a possible X-ray counterpart of the gravitational wave event.