# Parameter estimation of binary black holes in the endpoint of the up-down instability

**Authors:** Viola De Renzis, Davide Gerosa, Matthew Mould, Riccardo Buscicchio, Lorenzo Zanga  
**arXiv:** [2304.13063](https://arxiv.org/abs/2304.13063)  
**Date:** 2023-04-25  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2304.13063v1/

## Key points

- The study focuses on the up-down instability in black hole binary spin precession.
- Up-down instability occurs when the spins of two black holes are aligned or anti-aligned with each other and with the orbital angular momentum.
- Binaries in an up-down configuration may be unstable to small perturbations of their spin directions, leading to gravitational wave sources that formed with aligned spins but are detected with precessing spins.
- The authors present a Bayesian procedure based on the Savage-Dickey density ratio to test whether a given gravitational wave event originated from an up-down configuration.
- This procedure can be applied to both simulated signals and actual data from LIGO/Virgo events.
- Achieving strong evidence for an up-down origin is within reach of current experiments using simulated signals, but analysis of LIGO/Virgo events released to date indicates that current data are not informative enough for this purpose.
- This study sheds light on a previously unexplored aspect of black hole binary spin precession and provides a new tool for analyzing gravitational wave events.

## Summary

The study by Viola De Renzis, Davide Gerosa, Matthew Mould, Riccardo Buscicchio and Lorenzo Zanga focuses on the up-down instability in black hole binary spin precession. This phenomenon occurs when the spins of the two black holes are either aligned or anti-aligned with each other and with the orbital angular momentum. Binaries in an up-down configuration - where the heavier (lighter) black hole's spin is co-(counter-) aligned with the orbital angular momentum - may be unstable to small perturbations of their spin directions. The occurrence of this instability leads to gravitational wave sources that formed with aligned spins but are detected with precessing spins. To test whether a given gravitational wave event originated from an up-down configuration, the authors present a Bayesian procedure based on the Savage-Dickey density ratio. This procedure can be applied to both simulated signals and actual data from LIGO/Virgo events. The authors find that achieving strong evidence for an up-down origin is within reach of current experiments using simulated signals. However, analysis of LIGO/Virgo events released to date indicates that current data are not informative enough for this purpose. Overall, this study sheds light on a previously unexplored aspect of black hole binary spin precession and provides a new tool for analyzing gravitational wave events.