论文标题
AGN磁盘中紧密包装的恒星黑洞的长期演变：通过近距离接触形成黑洞二进制
Long-term Evolution of Tightly-Packed Stellar Black Holes in AGN Disks: Formation of Merging Black-Hole Binaries via Close Encounters
论文作者
论文摘要
我们研究了两个或多个恒星黑洞（BHS）的长期演变，该孔在超质量BH（SMBH）周围最初分离但不稳定的圆轨道上。这种近包装的轨道构型自然而然地来自AGN磁盘中的BH迁移。轨道的动态不稳定性导致两个BH之间的重复接近遇到，在此期间，BH分离$ r _ {\ rm p} $变成小于山地半径$ r _ {\ rm h} $。在罕见的非常接近的相遇（$ r _ {\ rm p} $几个数量级小于$ r _ {\ rm h} $）中，紧密合并的BH二进制可以在引力波排放的帮助下形成。我们使用$ n $ - 体仿真来研究各种“亲密”程度的紧密相遇和由此产生的二进制BH合并的属性的时间演变。对于典型的“ SMBH + 2 BHS”系统，平均累积数量的关闭遇到数（带有$ r _ {\ rm p} \ sillsim r _ {\ rm h} $）大约缩放为$ \ propto propto t^{0.5} $。最小遭遇分离$ r _ {\ rm p} $遵循线性累积分布$ p（<r _ {\ rm p}）\ propto r _ {\ rm p} $ for $ r _ {\ rm p}从这些情况下，我们获得了导致BH合并的平均二元捕获速率的半分析表达。我们的结果表明，AGN磁盘中的封闭式BHS可能需要很长时间（SMBH周围的$ \ gtrsim 10^7 $轨道）才能体验到足够紧密的相遇并形成绑定的二进制，尽管如果初始的BH轨道高度一致，则此时间可能会更短。在这种情况下产生的BH二进制合并在进入LIGO频段时总是具有很高的偏心率，并且相对于原始AGN磁盘具有广泛的轨道倾斜分布。我们还探索了气盘的效果，发现简单的气体在BHS上拖动不一定会导致BH二进制捕获率提高。
We study the long-term evolution of two or more stellar black holes (BHs) on initially separated but unstable circular orbits around a supermassive BH (SMBH). Such a close-packed orbital configuration can naturally arise from BH migrations in the AGN disk. Dynamical instability of the orbits leads to recurring close encounters between two BHs, during which the BH separation $r_{\rm p}$ becomes less than the Hill radius $R_{\rm H}$. In the rare very close encounters (with $r_{\rm p}$ several orders of magnitude less than $R_{\rm H}$), a tight merging BH binary can form with the help of gravitational wave emission. We use $N$-body simulations to study the time evolution of close encounters of various degrees of "closeness" and the property of the resulting binary BH mergers. For a typical "SMBH + 2 BHs" system, the averaged cumulative number of close encounters (with $r_{\rm p} \lesssim R_{\rm H}$) scales approximately as $\propto t^{0.5}$. The minimum encounter separation $r_{\rm p}$ follows a linear cumulative distribution $P(<r_{\rm p}) \propto r_{\rm p}$ for $r_{\rm p} \ll R_{\rm H}$. From these, we obtain a semi-analytical expression for the averaged rate of binary captures that lead to BH mergers. Our results suggest that close-packed BHs in AGN disks may take a long time ($\gtrsim 10^7$ orbits around the SMBH) to experience a sufficiently close encounter and form a bound binary, although this time can be shorter if the initial BH orbits are highly aligned. The BH binary mergers produced in this scenario always have high eccentricities when entering the LIGO band, and have a broad distribution of orbital inclinations relative to the original AGN disk. We also explore the effects of the gas disk and find that simple gas drags on the BHs do not necessarily lead to an enhanced BH binary capture rate.
