Repeated Mergers of Black Hole Binaries: Implications for GW190521

Abstract

The gravitational wave event GW190521 involves the merger of two black holes of ∼85 M ⊙ and ∼66 M ⊙ forming an intermediate-mass black hole (IMBH) of mass ∼142 M ⊙. Both progenitors are challenging to explain within standard stellar evolution as they are within the upper black hole mass gap. We propose a dynamical formation pathway for this IMBH based on multiple mergers in the core of a globular cluster. We identify such scenarios from analysis of a set of 58 N-body simulations using NBODY6-gpu. In one of our simulations, we observe a stellar black hole undergoing a chain of seven binary mergers within 6 Gyr, attaining a final mass of 97.8 M ⊙. We discuss the dynamical interactions that lea..