Binary black holes with spins that are aligned with the orbital angular momentum do not precess. However, post-Newtonian calculations predict that “up-down” binaries, in which the spin of the heavier (lighter) black hole is aligned (antialigned) with the orbital angular momentum, are unstable when the spins are slightly perturbed from perfect alignment. This instability provides a possible mechanism for the formation of precessing binaries in environments where sources are preferentially formed with (anti)aligned spins. In this paper, we present the first full numerical relativity simulations capturing this instability. These simulations span $\sim 100$ orbits and $\sim 3–5$ precession cycles before merger, making them some of the longest numerical relativity simulations to date. Initialized with a small perturbation of 1°–10°, the instability causes a dramatic growth of the spin misalignments, which can reach $\sim 90°$ near merger. We show that this leaves a strong imprint on the subdominant modes of the gravitational wave signal, which can potentially be used to distinguish up-down binaries from other sources. Finally, we show that post-Newtonian and effective-one-body approximants are able to reproduce the unstable dynamics of up-down binaries extracted from numerical relativity.

中文翻译：

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相对论中二元黑洞的上下不稳定性

自旋与轨道角动量对齐的二元黑洞不会进动。但是，牛顿后的计算预测，“自上而下”的二进制文件（其中较重（较轻）的黑洞的自旋与轨道角动量对齐（反对齐））在自旋受到完美对齐的轻微干扰时是不稳定的。这种不稳定性提供了一种可能的机制，用于在优先以（反）排列的自旋形成源的环境中形成进动二进制。在本文中，我们提出了捕获这种不稳定性的第一个完整的数值相对论模拟。这些模拟跨度$〜100$ 轨道和 $〜3–5$合并之前的旋进周期，使它们成为迄今为止最长的数值相对论模拟。最初以1°–10°的微小扰动开始，这种不稳定性会导致自旋未对准的急剧增加，可以达到$〜90°$接近合并。我们表明，这在重力波信号的主要模式上留下了深刻的烙印，可以将其潜在地用于区分其他来源的上下二进位。最后，我们表明后牛顿近似和有效一体近似能够重现从数值相对论中提取的上下二元的不稳定动力学。