The Legacy Survey of Space and Time (LSST) by the Vera C. Rubin Observatory is expected to discover tens of millions of quasars. A significant fraction of these could be powered by coalescing massive black hole (MBH) binaries, since many quasars are believed to be triggered by mergers. We show that under plausible assumptions about the luminosity functions, lifetimes, and binary fractions of quasars, we expect the full LSST quasar catalogue to contain between 20 and 100 million compact MBH binaries with masses M = 105–9M⊙, redshifts z = 0–6, and orbital periods P = 1–70 d. Their light-curves are expected to be distinctly periodic, which can be confidently distinguished from stochastic red-noise variability, because LSST will cover dozens, or even hundreds of cycles. A very small subset of 10–150 ultracompact (P ≲ 1 d) binary quasars among these will, over ∼5–15 yr, evolve into the mHz gravitational-wave frequency band and can be detected by LISA. They can therefore be regarded as ‘LISA verification binaries’, analogous to short-period Galactic compact-object binaries. The practical question is how to find these handful of ‘needles in the haystack’ among the large number of quasars: this will likely require a tailored co-adding analysis optimized for this purpose.

中文翻译：

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LSST中的超短周期大质量黑洞二进制候选作为LISA“验证二进制”

Vera C. Rubin 天文台的时空遗产调查 (LSST) 预计将发现数千万个类星体。其中很大一部分可以由合并的大质量黑洞 (MBH) 双星驱动，因为许多类星体被认为是由合并触发的。我们表明，在关于类星体的光度函数、寿命和二元分数的合理假设下，我们预计完整的 LSST 类星体目录将包含 20 到 1 亿个质量 M = 105–9M⊙，红移 z = 0– 6，轨道周期 P = 1-70 d。预计它们的光曲线将具有明显的周期性，可以自信地将其与随机红噪声变化区分开来，因为 LSST 将涵盖数十个甚至数百个周期。其中非常小的 10-150 个超致密（P ≲ 1 d）二元类星体将在 5-15 年内演化到 mHz 引力波频段，并且可以被 LISA 检测到。因此，它们可以被视为“LISA 验证双星”，类似于短周期银河致密天体双星。实际问题是如何在大量类星体中找到这些“大海捞针”：这可能需要针对此目的进行优化的定制共添加分析。