The astrophysical origin of gravitational wave (GW) events is one of the most timely problems in the wake of the LIGO/Virgo discoveries. In active galactic nuclei (AGN), binaries form and evolve efficiently by dynamical interactions and gaseous dissipation. Previous studies have suggested that binary black hole (BBH) mergers in AGN disks can contribute significantly to BBH mergers observed by GW interferometers. Here we examine the distribution of the effective spin parameter $\chi_\mathrm{eff}$ of this GW source population. We extend our semi-analytical model of binary formation and evolution in AGN disks by following the evolution of the binary orbital angular momenta and black hole (BH) spins. BH spins change due to gas accretion and BH mergers, while the binary orbital angular momenta evolve due to gas accretion and binary-single interactions. We find that the distribution of $\chi_\mathrm{eff}$ predicted by our AGN model is similar to the distribution observed during LIGO/Virgo O1 and O2. On the other hand, if radial migration of BHs is inefficient, $\chi_\mathrm{eff}$ is skewed toward higher values compared with the observed distribution, because of the paucity of scattering events that would randomize spin directions relative to the orbital plane. We suggest that high binary masses and the positive correlation between binary mass and the standard deviation of $\chi_\mathrm{eff}$ for chirp masses up to $\approx 20$ $\mathrm{M}_\odot$, can be possible signatures for mergers originating in AGN disks. Finally, hierarchical mergers in AGN disks naturally produce properties of the recent GW event GW190412, including a low mass ratio, a high primary BH spin, and a significant spin component in the orbital plane.

中文翻译：

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活动星系核中恒星质量黑洞双星的自旋演化

引力波 (GW) 事件的天体物理学起源是 LIGO/Virgo 发现之后最及时的问题之一。在活动星系核 (AGN) 中，双星通过动力相互作用和气体耗散有效地形成和演化。先前的研究表明，AGN 盘中的双黑洞 (BBH) 合并可以对 GW 干涉仪观察到的 BBH 合并做出重大贡献。在这里，我们检查了这个 GW 源群体的有效自旋参数 $\chi_\mathrm{eff}$ 的分布。我们通过跟踪二元轨道角动量和黑洞 (BH) 自旋的演化，扩展了 AGN 盘中二元形成和演化的半解析模型。BH 自旋因气体吸积和 BH 合并而变化，而二元轨道角动量因气体吸积和二元-单相互作用而演变。我们发现我们的 AGN 模型预测的 $\chi_\mathrm{eff}$ 的分布类似于 LIGO/Virgo O1 和 O2 期间观察到的分布。另一方面，如果 BHs 的径向迁移效率低下，则与观察到的分布相比，$\chi_\mathrm{eff}$ 偏向更高的值，因为很少有散射事件会使相对于轨道平面的自旋方向随机化. 我们建议高二元质量和二元质量与 $\chi_\mathrm{eff}$ 标准偏差之间的正相关性对于高达 $\approx 20$ $\mathrm{M}_\odot$ 的啁啾质量，可以是源自 AGN 磁盘的合并的可能签名。最后，AGN 盘中的分层合并自然产生了最近 GW 事件 GW190412 的特性，包括低质量比、高初级 BH 自旋、