Kilohertz Gravitational Waves from Binary Neutron Star Mergers: Inference of Postmerger Signals with the Einstein Telescope

Abstract

Next-generation detectors are expected to be sensitive to postmerger signals from binary neutron star coalescences and thus to directly probe the remnant dynamics. We investigate the scientific potential of postmerger detections with the Einstein Telescope using full Bayesian analyses with the state of the art waveform model NRPMw. We find that: (i) Postmerger signals with SNR ∼7 can be confidently detected with a Bayes' factor of log B 5 (e-folded) and the posterior distributions report informative measurements already at SNR ∼6 for some noise realizations. (ii) The postmerger peak frequency f2 can be confidently identified at SNR 7 with errors of O(1 kHz), that decrease below O(100 Hz) for SNR 10. (iii) The remnant's time of collapse to black hole can be constrained to O(20 ms) at SNR 10. However, the inference can be biased by noise fluctuations if the latter exceed the signal's amplitude before collapse. (iv) Violations of the EOS-insentive relations for f2 can be detected at SNR 8 if the frequency shifts are 500 Hz; they can be smoking guns for EOS softening effects at extreme densities. However, the f2 measurement can be significantly biased by subdominant frequency components for short-lived remnants. In these cases, an EOS softening might be better inferred from the remnant's earlier collapse.

Emergence of microphysical viscosity in binary neutron star post-merger dynamics

“…Critically, some of these constraints heavily rely on properties of the mass ejected after merger (e.g., [32][33][34][35][36][37][38]), which crucially depends on models for the post-merger evolution, including the lifetime of the remnant neutron star (see, e.g., [39][40][41] for recent reviews). Future detection of post-merger gravitational wave signals [42][43][44][45][46][47][48] have the potential for putting even more stringent constraints on the EoS [49][50][51][52][53][54][55][56][57][58].…”

What Can We Learn about the Unstable Equation-of-state Branch from Neutron Star Mergers?

“…This difference results in a shift of about 30 Hz in the GW peak frequency between the most extreme scenarios, for Γ = 0.25 and Γ = 2.50. This value is lower than the numerical uncertainty of our simulation, which we estimate to be around 70 Hz by performing simulations with a grid spacing of about 130 m. Previous studies revealed measurement uncertainties of the peak frequency of about 50-100 Hz for close BNS mergers for which the postmerger signal-to-noise ratio (S/N) would be of the order ( ) 10 (see, e.g., Clark et al 2016;Rezzolla & Takami 2016;Breschi et al 2022;or Branchesi et al 2023). We also point out that it would be extremely difficult to have a shift due to the unstable-branch EOS from other effects, such as magnetic fields or neutrino radiation.…”

Effects of a Late Gravitational Transition on Gravitational Waves and Anticipated Constraints

“…In specific variants of modified gravity theories, a discontinuity in the gravitational constant could engender a discrepancy between light propagation and gravitational waves. With the emergence of third-generation gravitational-wave observatories, for instance, the ET [77,81,117,123] or LISA [143][144][145][146], it will be feasible to directly examine scenarios encompassing gravitational transitions utilizing the luminosity distance d gw L of gravitational waves [81] and comparing it with the luminosity distance at the same redshift obtained with electromagnetic waves.…”