We estimate the merger timescale of spectroscopically selected, subparsec supermassive black hole binary (SMBHB) candidates by comparing their expected contribution to the gravitational-wave background (GWB) with the sensitivity of current pulsar timing array (PTA) experiments and in particular, with the latest upper limit placed by the North American Nanohertz Observatory for Gravitational Waves. We find that the average timescale to coalescence of such SMBHBs is ##IMG## [http://ej.iop.org/images/2041-8205/900/2/L42/apjlabb2abieqn1.gif] {$\langle {t}_{\mathrm{evol}}\rangle \gt 6\times {10}^{4}\,$} yr, assuming that their orbital evolution in the PTA frequency band is driven by emission of gravitational waves. If some fraction of SMBHBs do not reside in spectroscopically detected active galaxies, and their incidence in active and inactive galaxies is similar, then the merger timescale could be ∼10 times longer, ##IMG## {$\langle {t}_{\mathrm...}

中文翻译：

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亚秒级超大规模黑洞二元候选者合并时间尺度上的脉冲星时序阵列约束

我们通过比较他们对引力波背景（GWB）的预期贡献与当前脉冲星时序阵列（PTA）实验的灵敏度，尤其是与北美引力波纳米天文台设定的最新上限。我们发现，此类SMBHBs合并的平均时间尺度为## IMG ## [http://ej.iop.org/images/2041-8205/900/2/L42/apjlabb2abieqn1.gif] {$ \ langle {t } _ {\ mathrm {evol}} \ rangle \ gt 6 \ times {10} ^ {4} \，$}年，假设它们在PTA频带中的轨道演化是由引力波的发射驱动的。如果某些SMBHBs不在光谱检测到的活跃星系中，