Abstract
ABSTRACT GW190521 is the most massive binary black hole (BBH) merger observed to date, and its primary component lies in the pair-instability (PI) mass gap. Here, we investigate the formation of GW190521-like systems via three-body encounters in young massive star clusters. We performed 2 × 105 simulations of binary-single interactions between a BBH and a massive $\ge {60}\,$M⊙ black hole (BH), including post-Newtonian terms up to the 2.5 order and a prescription for relativistic kicks. In our initial conditions, we take into account the possibility of forming BHs in the PI mass gap via stellar collisions. If we assume that first-generation BHs have low spins, $\sim {0.17}{{\ \rm per\ cent}}$ of all the simulated BBH mergers have component masses, effective and precessing spin, and remnant mass and spin inside the $90{{\ \rm per\ cent}}$ credible intervals of GW190521. Seven of these systems are first-generation exchanged binaries, while five are second-generation BBHs. We estimate a merger rate density $\mathcal {R}_{\rm GW190521}\sim {0.03}\,$Gpc$^{-3}\,$yr−1 for GW190521-like binaries formed via binary-single interactions in young star clusters. This rate is extremely sensitive to the spin distribution of first-generation BBHs. Stellar collisions, second-generation mergers and dynamical exchanges are the key ingredients to produce GW190521-like systems in young star clusters.
Highlights
Since the detection of GW150914 (Abbott et al 2016a; Abbott et al 2016b), the number of gravitational wave (GW) sources observed by the LIGO–Virgo collaboration (LVC) has increased year after year, culminating with the recent publication of the results of the first half of the third LVC observing run (Abbott et al 2021b,c)
We simulated 2 × 105 three-body encounters between a binary black hole (BBH) and a single massive black hole (BH) using the direct N -body code arwv (Arca-Sedda & Capuzzo-Dolcetta 2019; Chassonnery et al 2019; Chassonnery & Capuzzo-Dolcetta 2021). arwv exploits the algorithmic regularization chain method to integrate the equations of motion (Mikkola & Aarseth 1989, 1993)
Our simulations indicate that GW190521 can be the result of a first-generation exchanged BBH with at least one component produced by a stellar merger, or of a second-generation BBH
Summary
Since the detection of GW150914 (Abbott et al 2016a; Abbott et al 2016b), the number of gravitational wave (GW) sources observed by the LIGO–Virgo collaboration (LVC) has increased year after year, culminating with the recent publication of the results of the first half of the third LVC observing run (Abbott et al 2021b,c). The primary BH of GW190521 has a 99% probability of lying in the pair-instability (PI) mass gap (∼ 60 − 120 M , Abbott et al 2020a,c, see Mehta et al 2021) In this mass range, no BH is expected to form from the collapse of a single star, as a consequence of the unstable oxygen-silicon burning phase experienced by the progenitor (Heger & Woosley 2002; Woosley et al 2007; Belczynski et al 2016; Spera & Mapelli 2017; Woosley 2017; Marchant et al 2019; Stevenson et al 2019; Woosley 2019; Woosley & Heger 2021). The primary mass would safely be above the upper edge of the mass gap
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