# Impact of ultralight bosonic dark matter on the dynamical bar-mode instability of rotating neutron stars

**Authors:** Fabrizio Di Giovanni, Nicolás Sanchis-Gual, Davide Guerra, Miquel Miravet-Tenét, Pablo Cerdá-Durán, José Antonio Font  
**arXiv:** [2206.00977](https://arxiv.org/abs/2206.00977)  
**Date:** 2022-06-02  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2206.00977v1/

## Key points

- Effects of ultralight bosonic field dark matter on unstable differentially-rotating neutron stars and their bar-mode instability dynamics
- Numerical simulations in general relativity to analyze rotating neutron stars with an initial spherically symmetric bosonic field cloud
- Bosonic field significantly modifies the development of bar-mode instability in neutron stars
- Modification depends on total mass of bosonic field and mass of boson particles
- Accreting bosonic field can suppress or quench dominant $\ell=m=2$ mode by forming a mixed (fermion-boson) star
- Mixed stars still undergo development of a mixed bar and emit significant gravitational waves
- Emission differs from that produced by isolated neutron stars without dark matter accretion
- Implications for ongoing searches for continuous gravitational waves, suggesting dark-matter accretion could alter expected frequency range
- Understanding and accounting for these effects crucial for accurately interpreting observational data and improving knowledge about astrophysical phenomena

## Summary

The effects of ultralight bosonic field dark matter on unstable differentially-rotating neutron stars and their bar-mode instability dynamics are investigated in this study. Numerical simulations in general relativity are performed to analyze rotating neutron stars that accrete an initial spherically symmetric bosonic field cloud. The simulations involve solving the Einstein-(complex, massive) Klein-Gordon-Euler and the Einstein-(complex) Proca-Euler systems. The results reveal that the presence of the bosonic field can significantly modify the development of the bar-mode instability in neutron stars. This modification depends on both the total mass of the bosonic field and the mass of the boson particles. Interestingly, in certain cases, the accreting bosonic field can even suppress or quench the dominant $\ell=m=2$ mode of bar-deformation by dynamically forming a mixed (fermion-boson) star which retains part of its original angular momentum. However, despite this suppression, it is found that these mixed stars still undergo a development of a mixed bar and emit significant gravitational waves. Importantly, this emission differs substantially from that produced by isolated neutron stars without dark matter accretion. These findings have implications for ongoing searches for continuous gravitational waves as they suggest that dark-matter accretion could alter or shift their expected frequency range. Therefore, understanding and accounting for such effects is crucial for accurately interpreting observational data and improving our knowledge about these astrophysical phenomena. Overall, this study highlights how ultralight bosonic dark matter can have a critical impact on rotating neutron stars' stability and their associated gravitational-wave emissions, providing valuable insights into potential modifications caused by dark matter accretion and emphasizing its importance in their study.