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.

预计在接下来的二十年内，将首次同时检测到与大质量黑洞 (MBH) 双星合并的电磁 (EM) 和引力波 (GW) 特征。通过将这个不断发展的领域扩展到低频 GW 区域，这些探测将开启多信使天体物理学的新时代，并将提供对 MBH 和星系演化的前所未有的理解。它们还将从根本上构成新的宇宙学探测器，并使独特的引力测试成为可能。这篇 Living Review 的目的是通过总结与 EM 对应物和 MBH 合并相关的关键发现、物理过程和想法来介绍该研究主题，因为它们在撰写本文时已为人所知。我们回顾了当前关于 MBH 双星的观测证据，讨论相关的物理过程和时间尺度，并总结在 MBH 合并的前驱、聚结和余辉阶段可能的 EM 对应 GW。我们还描述了开放性问题并讨论了这个充满活力和快节奏的研究领域的未来前景。