Gravitational synchronization in bosonic dark matter admixed neutron stars

Abstract

While the search for dark matter remains a central focus of modern astrophysics and high-energy physics, neutron stars provide natural laboratories in which the interaction between dark matter and baryonic matter can be studied. In this work we model dark matter as an ultralight bosonic field, which can accrete onto the neutron star and form a composite object bound through gravity. Using long-term, numerical relativity simulations in spherical symmetry, we extract and analyze the frequency spectra of the radial oscillation modes of fermion-boson stars. Our simulations reveal that the fermionic and bosonic components synchronize through gravitational coupling, enriching their oscillation spectrum. This synchronization leads to new multi-state scalar configurations and reshapes the hierarchy of the neutron-star radial modes. We further propose a procedure to compute the values of the new dominant modes as a function of the bosonic mass, and discuss the implications for neutron-star physics and gravitational-wave astronomy.

Figures (7)

• Figure 1: Evolution of the radial profiles of $|\Phi|$ (left) and Fourier spectra of the central value of $\rm Re(\Phi)$ (right) for the FBS formation with initial parameters $\rho_c=1.5\times10^{-3}$ and $A_{\Phi}=3.00\times10^{-4}$. Since our code uses geometric units based on solar masses, physical units are recovered using the conversion factor $1 M_\odot \approx 1.477\;\rm km \approx4.926\!\times\!10^{-6}\rm s.$
• Figure 2: FFT of the real and imaginary parts of $\Phi_c$ (top), and spacetime and matter terms that gravitationally synchronize (bottom). The time window used is $[32.7;41.7]\!\times\!10^{3}$ and the corresponding FFT frequency resolution is $\Delta\omega\approx0.7\!\times\!10^{-3}$.
• Figure 3: NS spectrum showing a resonance in the evolution of an excited equilibrium FBS with one node corresponding to $\rho_c=1.5\times10^{-3}$ and $\phi_{1,c}=1.9\times10^{-2}$.
• Figure 4: Spectrum of $\rm Re(\Phi_c)$ (Top) and the rest-mass fluid density $\rho_c$ (bottom) changing the bosonic particle mass. The time window used was $[33.0;42.0]\!\times\!10^{3}$.
• Figure 5: Top: Domain of existence of the ground and first-excited state frequencies for $\mu=1$. Solid and dashed lines correspond to ground state and first-excited state frequencies, respectively. The same color denotes to the same $\rho_c$ value in the domains. Bottom: Resonant frequency obtained as $\Omega_{\rm new}=\omega_0-\omega_1$ from the top panel for different values of $\rho_c$.