Prospects for detection of high energy electromagnetic counterparts to gravitational wave mergers of massive black holes

The next two decades are expected to open the door to the first coincident detections of electromagnetic (EM) and gravitational wave (GW) signatures associated with massive black hole (MBH) binaries heading for coalescence. These detections will launch a new era of multimessenger astrophysics by expanding this growing field to the low-frequency GW regime and will provide an unprecedented understanding of the evolution of MBHs and galaxies. They will also constitute fundamentally new probes of cosmology and would enable unique tests of gravity. In this talk, I will summarize the possible EM counterparts to GWs in the precursor, coalescence, and afterglow stages of a MBH merger and discuss prospects for their detection in the high energy band.