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Gravitational waves from primordial black holes passing by neutron stars: observational prospects for the Galactic center

Nicolas Esser, Juan García-Bellido, Peter Tinyakov

Abstract

We investigate the gravitational wave (GW) signals emitted by planetary-mass primordial black holes (PBHs) passing nearby or traversing neutron stars (NSs). While previous studies mainly focused on the detailed waveforms of the signals, we estimate the rate of PBH-NS gravitational-wave events originating from the Galactic center and compute the probability of detecting a signal over 10 years of LIGO-Virgo-KAGRA observations. We examine in detail the case of PBHs bound to NSs, focusing on eccentric orbits that give rise to repeated GW bursts emitted in correlated series, each burst corresponding to a periastron passage. Despite the enhancement from the large number of bursts produced by a single PBH-NS pair, the total number of signals produced in this way remains subdominant to those due to random unbound encounters of PBHs with NSs. We also find that both types of signals have a very small probability $P\lesssim 10^{-8}$ to be detected in a 10 year period.

Gravitational waves from primordial black holes passing by neutron stars: observational prospects for the Galactic center

Abstract

We investigate the gravitational wave (GW) signals emitted by planetary-mass primordial black holes (PBHs) passing nearby or traversing neutron stars (NSs). While previous studies mainly focused on the detailed waveforms of the signals, we estimate the rate of PBH-NS gravitational-wave events originating from the Galactic center and compute the probability of detecting a signal over 10 years of LIGO-Virgo-KAGRA observations. We examine in detail the case of PBHs bound to NSs, focusing on eccentric orbits that give rise to repeated GW bursts emitted in correlated series, each burst corresponding to a periastron passage. Despite the enhancement from the large number of bursts produced by a single PBH-NS pair, the total number of signals produced in this way remains subdominant to those due to random unbound encounters of PBHs with NSs. We also find that both types of signals have a very small probability to be detected in a 10 year period.
Paper Structure (12 sections, 18 equations, 3 figures)

This paper contains 12 sections, 18 equations, 3 figures.

Table of Contents

  1. Introduction
  2. GW signal and strain
  3. PBHs bound to neutron stars
  4. PBH Energy loss and orbit evolution
  5. Gravitational waves
  6. Dynamical friction
  7. PBH inspiral time
  8. PBH deviations
  9. Probability of signal detection
  10. Dynamical friction: direct mergers
  11. Gravitational waves: correlated signals
  12. Results and conclusions

Figures (3)

  • Figure 1: SNR of the GW signal in LVK O4 as a function of $r_p$ for a PBH orbiting a neutron star in an elliptic or hyperbolic orbit with eccentricity close to $1$. The neutron star radius $R=10$ km is displayed as a dashed vertical line. The PBH mass is fixed to $m=10^{-4}M_\odot$ and the distance of the PBH-NS system with respect to the Earth is $d=8.2$ kpc. The SNR scales as $\propto m/d$, so the result can be straightforwardly rescaled to any PBH mass and distance to the Earth.
  • Figure 2: Periastron distances of PBHs orbiting neutron stars in elliptic or hyperbolic orbits with eccentricity close to $1$, for which an $\mathrm{SNR}>10$ will be generated in LVK O4, as a function of the PBH mass. The distance of the PBH-NS systems with respect to the Earth is $d=8.2$ kpc.
  • Figure 3: Probability of detecting GW signals from PBH-NS pairs in the Galactic Center over 10 years of observation with LVK O4 sensitivity, using fiducial values $\rho_\text{DM}=10\text{ GeV}/\text{cm}^3$ and $\bar{v}=\sqrt{2}\times200\text{km}/\text{s}$ for the Galactic properties. The most likely signals to be detected correspond to PBHs on unbound orbits passing close to neutron stars (dotted red). Next are PBHs bound to neutron stars and losing energy via dynamical friction (dashed blue), followed by those losing energy via gravitational wave emission (plain green).