薄吸积盘中双星的气体驱动旋进
Gas-driven inspiral of binaries in thin accretion disks
摘要：
气体吸积到超大质量黑洞双星（SMBHB）上的数值研究通常限于环双星盘（CBD）比活动星系核（AGN）中的盘厚10-100倍的条件。这种差异源于技术限制，也源于对复制基准数值模型的出版偏见。在这里，我们提出了第一个系统的研究如何二的轨道演化与磁盘规模的高度变化。我们报告了三个关键结果：（1）双星轨道演化从热盘（长宽比~0.1）的外旋转变为更真实的冷、薄盘的内旋，临界长宽比~0.04，对应于轨道马赫数~25。(2)双星上的净扭矩来自于靠近黑洞的气体轨道的正扭矩和CBD内边缘的负扭矩之间的竞争，CBD的内边缘对于较薄的磁盘来说更密集。这导致越来越薄的磁盘上的二进制越来越负的净扭矩。(3)随着马赫数的增加，吸积速率受到适度的抑制。我们讨论了我们的结果可能会影响建模的纳赫兹引力波背景，以及估计的LISA合并事件率。
英文摘要：
Numerical studies of gas accretion onto supermassive black hole binaries (SMBHBs) have generally been limited to conditions where the circumbinary disk (CBD) is 10-100 times thicker than expected for disks in active galactic nuclei (AGN). This discrepancy arises from technical limitations, and also from publication bias toward replicating fiducial numerical models. Here we present the first systematic study of how the binary's orbital evolution varies with disk scale height. We report three key results: (1) Binary orbital evolution switches from outspiralling for warm disks (aspect ratio ~0.1), to inspiralling for more realistic cooler, thinner disks at a critical aspect ratio ~0.04, corresponding to orbital Mach number ~25. (2) The net torque on the binary arises from a competition between positive torque from gas orbiting close to the black holes, and negative torque from the inner edge of the CBD, which is denser for thinner disks. This leads to increasingly negative net torques on the binary for increasingly thin disks. (3) The accretion rate is modestly suppressed with increasing Mach number. We discuss how our results may influence modeling of the nano-Hz gravitational wave background, as well as estimates of the LISA merger event rate.
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