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ABSTRACT Tidal disruption events (TDEs) are routinely observed in quiescent galaxies, as stars from the nuclear star cluster are scattered into the loss cone of the central supermassive black hole (SMBH). TDEs are also expected to occur in active galactic nuclei (AGNs), due to scattering or orbital eccentricity pumping of stars embedded in the innermost regions of the AGN accretion disc. Encounters with embedded stellar-mass black holes (BH) can result in AGN μTDEs. AGN TDEs and μTDEs could therefore account for a fraction of observed AGN variability. Here, by performing scattering experiments with the few-body code SpaceHub, we compute the probability of AGN TDEs and μTDEs as a result of 3-body interactions between stars and binary BHs. We find that AGN TDEs are more probable during the early life of the AGNs, when rates are $$\sim (6\times 10^{-5}-5 \times 10^{-2}) (f_\bullet /0.01)\, \rm {AGN}^{-1}$$ yr−1 (where f• is the ratio between the number density of BHs and stars), generally higher than in quiescent galactic nuclei. By contrast, μTDEs should occur throughout the AGN lifetime at a rate of $$\sim (1\times 10^{-4} - 4\times 10^{-2})(f_\bullet /0.01)\, \rm {AGN}^{-1}$$ yr−1. Detection and characterization of AGN TDEs and μAGN TDEs with future surveys using Rubin and Roman will help constrain the populations of stars and compact objects embedded in AGN discs, a key input for the LVK AGN channel.
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Binary black hole merger rates in AGN discs versus nuclear star clusters: loud beats quiet
ABSTRACT Galactic nuclei are promising sites for stellar origin black hole (BH) mergers, as part of merger hierarchies in deep potential wells. We show that binary black hole (BBH) merger rates in active galactic nuclei (AGNs) should always exceed merger rates in quiescent galactic nuclei (nuclear star clusters, NSCs) around supermassive black holes (SMBHs) without accretion discs. This is primarily due to average binary lifetimes in AGNs that are significantly shorter than those in NSCs. The lifetime difference comes from rapid hardening of BBHs in AGNs, such that their semimajor axes are smaller than the hard–soft boundary of their parent NSC; this contrasts with the large average lifetime to merger for BBHs in NSCs around SMBHs, due to binary ionization mechanisms. Secondarily, merger rates in AGNs are enhanced by gas-driven binary formation mechanisms. Formation of new BHs in AGN discs is a minor contributor to the rate differences. With the gravitational wave detection of several BBHs with at least one progenitor in the upper mass gap, and signatures of dynamical formation channels in the χeff distribution, we argue that AGNs could contribute $$\sim 25{\!-\!}80{{\ \rm per\ cent}}$$ of the LIGO–Virgo measured rate of $$\sim 24\, \rm {Gpc}^{-3} \rm {yr}^{-1}$$.
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- PAR ID:
- 10379817
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 517
- Issue:
- 4
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 5827-5834
- Size(s):
- p. 5827-5834
- Sponsoring Org:
- National Science Foundation
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