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Many black holes (BHs) detected by the Laser InterferometerGravitational-wave Observatory (LIGO) and the Virgo detectors are multipletimes more massive than those in X-ray binaries. One possibility is that someBBHs merge within a few Schwarzschild radii of a supermassive black hole(SMBH), such that the gravitational waves (GWs) are highly redshifted, causingthe mass inferred from GW signals to appear higher than the real mass. Thedifficulty of this scenario lies in the delivery of BBH to such a smalldistance to a SMBH. Here we revisit the theoretical models for the migration ofcompact 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 describedby the slim disc model, the COs in the disc could migrate to a radius close tothe innermost stable circular orbit (ISCO) and be trapped there for theremaining lifetime of the AGN. The exact trapping radius coincides with thetransition region between the sub- and super-Keplerian rotation of the slimdisc. We call this region "the last migration trap" because inside it COs canno longer be trapped for a long time. We pinpoint the parameter space whichcould induce such a trap and we estimate that the last migration trapcontributes a few per cent of the LIGO/Virgo events. Our result implies that acouple of BBHs discovered by LIGO/Virgo could have smaller intrinsic masses.

The last migration trap of compact objects in AGN accretion disc