Inspired by the General Relativity for many decades, experimental physicists and astronomers have a solid dream to detect gravitational waves (GWs) from mergers of black holes, which came true until the excellent performance of the Laser Interferometer Gravitational-Wave Observatory (LIGO) at hundreds Hz. Nano-Hz GWs are expected to be radiated by close-binaries of supermassive black holes (CBSMBHs; defined as those with separations less than ∼0.1 pc) formed during galaxy mergers and detected through the Pulsar Timing Array (PTA) technique. As of the writing, there remains no nano-Hz GWs detection. Searching for CB-SMBHs are also observationally elusive though there exist a number of possible candidates. In this review, we focus on observational signatures of CB-SMBHs from theoretic expectations, simulations and observations. These signatures appears in energy distributions of multiwavelength continuum, long term variations of continuum, jet morphology, reverberation delay maps and spectroastrometry of broad emission lines, AGN type transitions between type-1 and type-2 (changing-look), and gaseous dynamics of circumbinary disks, etc. Unlike hundred-Hz GWs from stellar mass black hole binaries, the waveform chirping of nano-Hz GWs is too slow to detect in a reasonable human timescale. We have to resort to electromagnetic observations to measure orbital parameters of CB-SMBHs in order to test nano-Hz GW properties. Reverberation mapping is a powerful tool for probing kinematics and geometry of ionized gas in the gravitational well of SMBHs (single or binary) and therefore provides a potential way to determine orbital parameters of CB-SMBHs. In particular, a combination of reverberation mapping with spectroastrometry (realized at the Very Large Telescope Interferometer) will further reinforce this capability. The Atacama Large Millimeter/submillimeter Array (ALMA) as well as the forthcoming Square Kilometer Array (SKA) are suggested to reveals dynamics of circumbinray disks through molecular emission lines.

中文翻译：

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活动星系核中超大质量黑洞的近双星观测特征

几十年来，受广义相对论的启发，实验物理学家和天文学家有一个坚实的梦想，即探测黑洞合并产生的引力波 (GW)，直到激光干涉引力波天文台 (LIGO) 在数百个赫兹。Nano-Hz GWs 预计会被超大质量黑洞 (CBSMBHs; 定义为那些在星系合并期间形成并通过脉冲星定时阵列 (PTA) 技术检测到的间隔小于 0.1 pc 的紧密双星体辐射。在撰写本文时，仍然没有纳米赫兹 GW 检测。尽管存在许多可能的候选者，但在观察上也难以寻找 CB-SMBH。在这篇综述中，我们从理论预期、模拟和观察中关注 CB-SMBH 的观察特征。这些特征出现在多波长连续谱的能量分布、连续谱的长期变化、射流形态、混响延迟图和宽发射线的光谱测量、类型 1 和类型 2 之间的 AGN 类型转变（变化外观）以及气体动力学环绕双星盘等。与来自恒星质量黑洞双星的百赫兹引力波不同，纳赫兹引力波的波形啁啾太慢而无法在合理的人类时间尺度内检测到。我们必须借助电磁观测来测量 CB-SMBH 的轨道参数，以测试纳米赫兹 GW 的特性。混响映射是探测 SMBH（单一或二元）引力井中电离气体运动学和几何学的有力工具，因此提供了一种确定 CB-SMBH 轨道参数的潜在方法。特别是，混响映射与光谱天体测量（在甚大望远镜干涉仪上实现）的结合将进一步加强这种能力。阿塔卡马大型毫米/亚毫米阵列 (ALMA) 以及即将推出的平方公里阵列 (SKA) 被建议通过分子发射线揭示环二进制射线盘的动力学。