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. The aim of this Living Review is to provide an introduction to this research topic by presenting a summary of key findings, physical processes and ideas pertaining to EM counterparts to MBH mergers as they are known at the time of this writing. We review current observational evidence for close MBH binaries, discuss relevant physical processes and timescales, and summarize the possible EM counterparts to GWs in the precursor, coalescence, and afterglow stages of a MBH merger. We also describe open questions and discuss future prospects in this dynamic and quick-paced research area.

预计未来二十年将为首次同时检测到与大质量黑洞（MBH）双星合并相关的电磁（EM）和引力波（GW）特征打开大门。这些探测将把这个不断发展的领域扩展到低频引力波体系，从而开启多信使天体物理学的新时代，并将为MBH和星系的演化提供前所未有的理解。它们还将构成全新的宇宙学探测器，并使独特的引力测试成为可能。这篇动态评论的目的是通过总结与 MBH 合并的 EM 对应物相关的主要发现、物理过程和想法来介绍该研究主题，正如在撰写本文时已知的那样。我们回顾了当前对接近的MBH双星的观测证据，讨论了相关的物理过程和时间尺度，并总结了MBH合并的前兆、合并和余辉阶段中GW可能的EM对应物。我们还描述了这个充满活力和快节奏的研究领域的开放性问题并讨论了未来的前景。