It is as well of wide interest and diverse implications (e.g. Abadie et al. 2010; Mandel & Farmer 2017) to consider how and under which conditions NSs and BHs would pair up in tight-enough binaries so that they can spiral in by emitting GW radiation and merge within the Hubble time. Recent numerical studies based on analytical (Hénon 1975), direct N-body integration (Aarseth 2003), and Monte Carlo approach (Hénon 1971; Joshi, Rasio & Portegies Zwart 2000; Hypki & Giersz 2013) show that the retention of BHs in dense stellar clusters of wide mass range, beginning from low-/medium-mass young and open clusters (e.g. Banerjee, Baumgardt & Kroupa 2010; Ziosi et al. 2014; Mapelli 2016; Banerjee 2017, 2018a, b; Park et al. 2017; Di Carlo et al. 2019; Kumamoto, Fujii & Tanikawa 2019; Rastello et al. 2019) through globular clusters (e.g. Breen & Heggie 2013; Morscher et al. 2013; Sippel & Hurley 2013; Arca-Sedda 2016; Hurley et al. 2016; Rodriguez, Chatterjee & Rasio 2016; Wang et al. 2016; Askar et al. 2017; Chatterjee, Rodriguez & Rasio 2017a; Chatterjee et al. 2017b; Fragione & Kocsis 2018; Rodriguez et al. 2018; Antonini & Gieles 2020; Kremer et al. 2020) to galactic nuclear clusters (e.g. Antonini & Rasio 2016; Hoang et al. 2018, 2019; Antonini, Gieles & Gualandris 2019; Arca-Sedda & Capuzzo-Dolcetta 2019; Arca Sedda 2020), comprise environments where BHs can pair up through close dynamical interactions, which, furthermore, lead to general-relativistic (hereafter GR) coalescences of these BBHs. Being much more massive than the rest of the stars, the BHs, which remain gravitationally bound to a cluster after their birth, spatially segregate and remain highly concentrated in the cluster’s innermost (and densest) region (e.g. Banerjee et al. 2010; Morscher et al. 2015) due to dynamical friction (Chandrasekhar 1943; Spitzer 1987) from the stellar background. This is essentially an early core collapse of the cluster leading to its post-core-collapse behaviour (Hénon 1975; Spitzer 1987; Heggie & Hut 2003), i.e. energy generation in the ‘collapsed’ BH core leading to an overall expansion of the cluster with time (Breen & Heggie 2013; Antonini & Gieles 2020). Inside this core, BHs undergo close binary–single and binary–binary encounters giving rise to compact subsystems (triples, quadruples, or even higher order multiples) whose resonant evolution can lead to GR inspiral and merger of their innermost binaries (Leigh & Geller 2013; Samsing, MacLeod & Ramirez-Ruiz 2014; Geller & Leigh 2015; Banerjee 2018b; Samsing 2018; Zevin et al. 2019), through the binaries’ eccentricity pumping. The breakup of such subsystems or simply close, flyby encounters may also lead to a sufficient boost in eccentricity of a BBH such that it merges either promptly, in between two close encounters (e.g. Kremer et al. 2019) or within a Hubble time if it gets ejected from the cluster as a result of the interaction (e.g. Rodriguez et al. 2015; Park et al. 2017; Kumamoto et al. 2019). Note that such GR mergers can also happen in hierarchical systems, containing NSs and BHs, in a galactic field that derive from field massive-stellar multiplets (e.g. Toonen, Hamers & Portegies Zwart 2016; Antonini, Toonen & Hamers 2017; Fragione & Loeb 2019; Fragione, Loeb & Rasio 2020b).
Citation: Kremer et al. 2020
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