AGN吸积盘中致密天体的最后一个迁移陷阱
The last migration trap of compact objects in AGN accretion disc
摘要：
激光干涉引力波天文台（LIGO）和室女座探测器探测到的许多黑洞（BH）的质量比X射线双星中的黑洞大好几倍。一种可能性是，一些黑洞在超大质量黑洞（SMBH）的几个史瓦西半径内合并，使得引力波（GW）高度红移，导致从GW信号推断的质量似乎高于实际质量。这种情况的困难在于BBH到SMBH的距离如此之小。在这里，我们回顾了活动星系核（AGN）的吸积盘中的紧凑对象（CO）的迁移的理论模型。我们发现，当吸积率很高，使光盘是最好的描述由薄盘模型，在光盘中的CO可以迁移到一个半径接近最内稳定圆轨道（ISCO），并被困在那里的AGN的剩余寿命。准确的捕获半径与细长盘的次开普勒旋转和超开普勒旋转之间的过渡区域相吻合。我们称这个区域为“最后的迁移陷阱”，因为在它里面，CO不再能被长时间地捕获。我们查明的参数空间，可能会导致这样的陷阱，我们估计，最后的迁移陷阱贡献了百分之几的LIGO/处女座事件。我们的结果意味着LIGO/Virgo发现的几个BBH可能具有较小的固有质量。
英文摘要：
Many black holes (BHs) detected by the Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo detectors are multiple times more massive than those in X-ray binaries. One possibility is that some BBHs merge within a few Schwarzschild radii of a supermassive black hole (SMBH), such that the gravitational waves (GWs) are highly redshifted, causing the mass inferred from GW signals to appear higher than the real mass. The difficulty of this scenario lies in the delivery of BBH to such a small distance to a SMBH. Here we revisit the theoretical models for the migration of compact objects (COs) in the accretion discs of active galactic nuclei (AGNs). We find that when the accretion rate is high so that the disc is best described by the slim disc model, the COs in the disc could migrate to a radius close to the innermost stable circular orbit (ISCO) and be trapped there for the remaining lifetime of the AGN. The exact trapping radius coincides with the transition region between the sub- and super-Keplerian rotation of the slim disc. We call this region "the last migration trap" because inside it COs can no longer be trapped for a long time. We pinpoint the parameter space which could induce such a trap and we estimate that the last migration trap contributes a few per cent of the LIGO/Virgo events. Our result implies that a couple of BBHs discovered by LIGO/Virgo could have smaller intrinsic masses.
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