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#### Hints of spin-orbit resonances in the binary black hole population

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2107.09693.pdf

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PhysRevLett.128.031101.pdf

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##### Citation

Varma, V., Biscoveanu, S., Isi, M., Farr, W. M., & Vitale, S. (2022). Hints of
spin-orbit resonances in the binary black hole population.* Physical Review Letters,* *128*(3): 031101. doi:10.1103/PhysRevLett.128.031101.

Cite as: https://hdl.handle.net/21.11116/0000-0009-E875-E

##### Abstract

Binary black hole spin measurements from gravitational wave observations can

reveal the binary's evolutionary history. In particular, the spin orientations

of the component black holes within the orbital plane, $\phi_1$ and $\phi_2$,

can be used to identify binaries caught in the so-called spin-orbit resonances.

In a companion paper, we demonstrate that $\phi_1$ and $\phi_2$ are best

measured near the merger of the two black holes. In this work, we use these

spin measurements to provide the first constraints on the full six-dimensional

spin distribution of merging binary black holes. In particular, we find that

there is a preference for $\Delta \phi = \phi_1 - \phi_2 \sim \pm \pi$ in the

population, which can be a signature of spin-orbit resonances. We also find a

preference for $\phi_1 \sim -\pi/4$ with respect to the line of separation near

merger, which has not been predicted for any astrophysical formation channel.

However, the strength of these preferences depends on our prior choices, and we

are unable to constrain the widths of the $\phi_1$ and $\Delta \phi$

distributions. Therefore, more observations are necessary to confirm the

features we find. Finally, we derive constraints on the distribution of recoil

kicks in the population, and use this to estimate the fraction of merger

remnants retained by globular and nuclear star clusters. We make our spin and

kick population constraints publicly available.

reveal the binary's evolutionary history. In particular, the spin orientations

of the component black holes within the orbital plane, $\phi_1$ and $\phi_2$,

can be used to identify binaries caught in the so-called spin-orbit resonances.

In a companion paper, we demonstrate that $\phi_1$ and $\phi_2$ are best

measured near the merger of the two black holes. In this work, we use these

spin measurements to provide the first constraints on the full six-dimensional

spin distribution of merging binary black holes. In particular, we find that

there is a preference for $\Delta \phi = \phi_1 - \phi_2 \sim \pm \pi$ in the

population, which can be a signature of spin-orbit resonances. We also find a

preference for $\phi_1 \sim -\pi/4$ with respect to the line of separation near

merger, which has not been predicted for any astrophysical formation channel.

However, the strength of these preferences depends on our prior choices, and we

are unable to constrain the widths of the $\phi_1$ and $\Delta \phi$

distributions. Therefore, more observations are necessary to confirm the

features we find. Finally, we derive constraints on the distribution of recoil

kicks in the population, and use this to estimate the fraction of merger

remnants retained by globular and nuclear star clusters. We make our spin and

kick population constraints publicly available.