Gravitational-wave asteroseismology with fundamental modes from compact binary inspirals

Geraint Pratten,Patricia Schmidt,Tanja Hinderer

doi:10.1038/s41467-020-15984-5

Abstract

Gravitational waves (GWs) from binary neutron stars encode unique information about ultra-dense matter through characterisic signatures associated with a variety of phenomena including tidal effects during the inspiral. The main tidal signature depends predominantly on the equation of state (EoS)-related tidal deformability parameter Λ, but at late times is also characterised by the frequency of the star’s fundamental oscillation mode (f-mode). In General Relativity and for nuclear matter, Λ and the f-modes are related by universal relations which may not hold for alternative theories of gravity or exotic matter. Independently measuring Λ and the f-mode frequency enables tests of gravity and the nature of compact binaries. Here we present directly measured constraints on the f-mode frequencies of the companions of GW170817. We also show that future GW detector networks will measure f-mode frequencies to within tens of Hz, enabling precision GW asteroseismology with binary inspiral signals alone.

Highlights

Gravitational waves (GWs) from binary neutron stars encode unique information about ultradense matter through characterisic signatures associated with a variety of phenomena including tidal effects during the inspiral
As an example of this concept, the coloured solid curves correspond to predictions from equation of state (EoS) models for neutron star (NS) with increasing stiffness known as APR419, MPA120, and H421 where the f-mode was calculated from universal relations (URs); as an example of an exotic object we show predictions for nonrotating boson stars[22,23]
The implications of these results are: (i) hyper-excited dynamical tides, i.e., anomalously small f-mode frequencies, are disfavoured by GW170817, (ii) current GW detections only allow for lower bounds to be placed on the quadrupolar and octupolar f-mode frequencies, (iii) the lower bounds are consistent with the predictions from URs for nuclear EoSs but do depend on the choice of the upper limit of the prior

Summary

Discussion

We present direct constraints on the fundamental oscillation modes in GW170817 using a waveform model with explicit dynamical tides and without assuming URs, demonstrating how we can directly measure the fundamental oscillation mode frequencies from the GW data alone. The highly localised PDF in the Λ-f plane anticipated with the future networks will help break degeneracies and confront the predictions from URs with stringent observational tests, enabling to discriminate between exotic compact objects and conventional nuclear matter models It will help constrain alternative theories of gravity and exotic physics beyond the standard model such as axions. By accurately measuring the f-mode frequency and mass of a neutron star, we can infer the moment-of-inertia I and the radius of the star, so long as the EoS does not exhibit significant softening at super-nuclear densities[6,41,42] This would enable us to probe matter at densities well above the nuclear saturation density[41], making gravitational-wave asteroseismology a potentially powerful tool in constraining the EoS of compact objects and probing the physics of their interiors. The analysis and methods in this paper lay the foundation for opening unique prospects for deriving fundamental information from compact binary inspirals

Methods

Findings

Code availability