Numerical Study of Spin and Mass Changes in Hyperbolic Black Hole Encounters
A new numerical relativity study published in Physical Review D finds that black holes can gain up to 15% in mass and increase their spin parameter by up to 0.3 during hyperbolic (non-merging) encounters, by reabsorbing energy and angular momentum radiated as gravitational waves. The research simulated equal-mass black hole pairs across a wide range of initial spins and approach trajectories, mapping the boundary between scattering and merging outcomes. These findings refine our understanding of black hole dynamics in dense stellar environments where close flybys are expected to occur.
Researchers have used numerical relativity simulations to systematically study how spinning black holes are altered during hyperbolic encounters — close gravitational flybys that do not result in a merger. The study, now published in Physical Review D, examined equal-mass black hole pairs with initial dimensionless spins ranging from -0.7 to 0.7, aligned or anti-aligned with the orbital angular momentum, and varied both the initial momenta and the angle of approach. The simulations show that spin-up and mass-gain are most pronounced near the threshold angle separating scattering from merging trajectories, at large momenta, and when initial spins are negative (anti-aligned). Across all configurations, the maximum observed spin-up was 0.3 and the maximum mass increase was 15%, both arising from the reabsorption of gravitational wave energy and angular momentum. A counterintuitive result emerged for highly spinning systems: black holes with an initial spin of χ = 0.7 can experience a net spin-down of the dimensionless spin parameter despite gaining angular momentum, because the accompanying mass gain is proportionally larger. The threshold angle between scattering and merging was also mapped across the parameter space, providing a useful reference for gravitational wave source modeling. These results are relevant for understanding black hole populations in globular clusters and galactic nuclei, where repeated close encounters are plausible.
What's missing
The study focuses exclusively on equal-mass, aligned/anti-aligned spin configurations; unequal-mass ratios and generic (precessing) spin orientations are not explored and may yield different spin-up and mass-gain behaviors. The astrophysical rate at which such hyperbolic encounters occur in realistic environments (globular clusters, galactic nuclei) is not estimated, leaving the observational relevance of these effects unquantified. Gravitational wave signal morphology from these scattering events and detectability by current or future detectors (LIGO/Virgo/LISA) are not addressed.
What different sources said
- arXiv astro-phCenter
Spin-up and mass-gain in hyperbolic encounters of spinning black holes
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