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# GW190521 as a dynamical capture of two nonspinning black holes

## Abstract

Gravitational waves from ~90 black hole binary systems have been detected and their progenitors' properties inferred1 so far by the Laser Interferometer Gravitational-Wave Observatory2 and Virgo3 experiments. This has 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 (refs. 4,5). We analyse this signal under the hypothesis that it was generated by the merger of two nonspinning black holes on hyperbolic orbits. The configuration best matching the data corresponds to two black holes of source-frame masses of $$8{1}_{-25}^{+62}{M}_{\odot }$$ and $$5{2}_{-32}^{+32}{M}_{\odot }$$ undergoing two encounters and then merging into an intermediate-mass black hole. We find that the hyperbolic merger hypothesis is favoured with respect to a quasi-circular merger with precessing spins with Bayes' factors larger than 4,300 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.

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## Data availability

The data are available on Zenodo at https://doi.org/10.5281/zenodo.7081337.

## Code availability

The eccentric waveform model used in this work, TEOBResumS, is publicly available at https://bitbucket.org/eob_ihes/teobresums/ and results presented in this paper have been obtained with the version tagged eccentric.v0_a6c_c3_circularized. TEOBResumSP is publicly available at the same address, and results presented here have been obtained with the version having git hash 56f20ad.

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## Acknowledgements

We are grateful to T. Damour, J. A. Font and T. Andrade for discussions. We are also grateful to D. Chiaramello for collaboration at the beginning of the project. We thank B. Daszuta, F. Zappa, W. Cook and D. Radice for supporting the development of the GR–Athena++ code and for help with the NR simulations presented in the Supplementary Information. R.G. acknowledges support from the Deutsche Forschungsgemeinschaft (DFG) under Grant No. 406116891 within the Research Training Group RTG 2522/1. M.B. and S.B. acknowledge support by the EU H2020 under ERC Starting Grant no. BinGraSp-714626. M.B. acknowledges partial support from the DFG under Grant No. 406116891 within the Research Training Group RTG 2522/1. G.C. acknowledges support by the Della Riccia Foundation under an Early Career Scientist Fellowship. Computations were performed on the national Hewlett Packard Enterprise Apollo Hawk at the High Performance Computing Center Stuttgart (HLRS), on the ARA cluster at Friedrich Schiller University Jena and on the Tullio sever at INFN Turin. The ARA cluster is funded in part by DFG grants INST 275/334-1 FUGG and INST 275/363-1 FUGG and by the ERC Starting Grant, grant agreement no. BinGraSp-714626. The authors acknowledge HLRS for funding this project by providing access to the supercomputer HPE Apollo Hawk under the grant number INTRHYGUE/44215. We thank E. Ferrari for speed-up coding work on Tullio. This research has made use of data, software and/or web tools obtained from the Gravitational Wave Open Science Center (https://www.gw-openscience.org), a service of LIGO Laboratory, the LIGO Scientific Collaboration and the Virgo Collaboration. LIGO is funded by the US National Science Foundation. Virgo is funded by the French Centre National de Recherche Scientifique (CNRS), the Italian Istituto Nazionale della Fisica Nucleare (INFN) and the Dutch Nikhef, with contributions by Polish and Hungarian institutes.

## Author information

Authors

### Contributions

S.B. and A.N. contributed to the origination of the idea. A.N., P.R., R.G. and S.B. developed and tested the waveform model. R.G., M.B. and G.C. performed the analyses and S.A. carried out numerical relativity simulations. R.G. produced all the figures. All authors worked out collaboratively the general details of the project. All authors helped edit the manuscript.

### Corresponding author

Correspondence to A. Nagar.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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Nature Astronomy thanks the anonymous reviewers for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–4, Tables 1–3 and supplementary discussion.

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Gamba, R., Breschi, M., Carullo, G. et al. GW190521 as a dynamical capture of two nonspinning black holes. Nat Astron 7, 11–17 (2023). https://doi.org/10.1038/s41550-022-01813-w

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• DOI: https://doi.org/10.1038/s41550-022-01813-w