GW190521 as a dynamical capture of two nonspinning black holes

TL;DR

This paper proposes that the gravitational wave event GW190521 resulted from a hyperbolic encounter and merger of two nonspinning black holes, challenging the typical circular orbit assumption and suggesting a dynamical capture origin.

Contribution

It introduces the hypothesis that GW190521 was generated by a hyperbolic encounter of nonspinning black holes, supported by Bayesian analysis favoring this scenario over circular mergers.

Findings

Hyperbolic merger hypothesis is strongly favored over circular models.

Estimated black hole masses involved are approximately 81 and 52 solar masses.

Results suggest GW190521 could be the first detection of a dynamical capture event.

Abstract

Gravitational waves from $\sim 90$ black holes binary systems have currently been detected by the LIGO and Virgo experiments, and their progenitors' properties inferred. This allowed the scientific community to draw conclusions on the formation channels of black holes in binaries, informing population models and -- at times -- defying our understanding of black hole astrophysics. The most challenging event detected so far is the short duration gravitational-wave transient GW190521. We analyze this signal under the hypothesis that it was generated by the merger of two nonspinning black holes on hyperbolic orbits. The best configuration matching the data corresponds to two black holes of source frame masses of $81^{+62}_{-25}M_\odot$ and $52^{+32}_{-32}M_\odot$ undergoing two encounters and then merging into an intermediate-mass black hole. We find that the hyperbolic merger hypothesis is favored with respect to a quasi-circular merger with precessing spins with Bayes' factors larger than 4300 to 1, although this number will be reduced by the currently uncertain prior odds. Our results suggest that GW190521 might be the first gravitational-wave detection from the dynamical capture of two stellar-mass nonspinning black holes.