Accretion discs in active galactic nuclei (AGNs) foster black hole (BH) formation, growth, and mergers. Stellar mass BHs migrate inwards under the influence of hydrodynamical torques unless they encounter a region where the torque flips sign. At these migration traps, BHs accumulate and merge via dynamical or gas-assisted interactions, producing high-frequency LIGO/Virgo/KAGRA (LVK) gravitational wave (GW) sources and potentially cutting off the supply of extreme mass ratio inspirals that would otherwise make low-frequency, LISA-band GWs. In this paper, we study the interplay between different types of migration torques, focusing especially on the ‘thermal torques’ generated by the thermal response of the AGN to embedded stellar-mass BHs that accrete through their own mini-discs. In contrast to previous work, we find that Type I torques cannot produce migration traps on their own, but thermal torques often do, particularly in low-mass AGN. The migration traps produced by thermal torques exist at much larger distances (∼103−5 gravitational radii) than do previously identified Type I traps, carrying implications for GW populations at multiple frequencies. Finally, we identify a bifurcation of AGN discs into two regimes: migration traps exist below a critical AGN luminosity, and do not at higher luminosities. This critical luminosity is fit as $\log _{10} L_{\rm AGN}^c = 45 {\!-\!} 0.32 \log _{10}{(\alpha /0.01)}$ where α is the Shakura–Sunyaev viscosity parameter, a range compatible with recent claims that LVK GWs are not preferentially associated with high-luminosity AGN.

中文翻译：

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热扭矩对活动星系核盘迁移陷阱和引力波群的影响

活跃星系核 (AGN) 中的吸积盘促进黑洞 (BH) 的形成、生长和合并。恒星质量 BH 在流体动力扭矩的影响下向内迁移，除非它们遇到扭矩翻转符号的区域。在这些迁移陷阱中，BH 通过动力或气体辅助相互作用积累和合并，产生高频 LIGO/Virgo/KAGRA (LVK) 引力波 (GW) 源，并可能切断极端质量比吸气的供应，否则会导致低频 LISA 频段 GW。在本文中，我们研究了不同类型的迁移扭矩之间的相互作用，特别关注活动星系核对嵌入的恒星质量黑洞的热响应所产生的“热扭矩”，这些恒星质量黑洞通过它们自己的迷你盘吸积。与之前的工作相比，我们发现 I 型扭矩本身不能产生迁移陷阱，但热扭矩经常会产生迁移陷阱，特别是在低质量活动星系核中。由热扭矩产生的迁移陷阱存在的距离（~103−5 引力半径）比之前识别的 I 型陷阱大得多，对多个频率的引力波种群具有影响。最后，我们将 AGN 盘分为两种状态：迁移陷阱存在于 AGN 临界光度以下，而在较高光度下则不存在。该临界光度拟合为 $\log _{10} L_{\rm AGN}^c = 45 {\!-\!} 0.32 \log _{10}{(\alpha /0.01)}$ 其中 α 是Shakura-Sunyaev 粘度参数，这个范围与最近的说法一致，即 LVK GW 不优先与高光度 AGN 相关。