GW190521 formation via three-body encounters in young massive star clusters

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$\times10^5$ simulations of binary-single interactions between a BBH and a massive $\geq{60}\,$M${\odot}$ 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, $\sim{0.17}\%$ of all the simulated BBH mergers have component masses, effective and precessing spin, and remnant mass and spin inside the $90\%$ 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.