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Kilohertz gravitational waves from binary neutron star mergers. II. Inference of postmerger signals with the Einstein Telescope

Matteo Breschi, Rossella Gamba, Gregorio Carullo, Ssohrab Borhanian, and Sebastiano Bernuzzi

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Phys. Rev. D 112, 124001 – Published 1 December, 2025

DOI: https://doi.org/10.1103/nmns-md9c

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 observations with the Einstein Telescope using full Bayesian analyses with the state of the art waveform model NRPMw. We show that postmerger signals with SNR∼7 can be confidently detected with a Bayes’ factor of log B≃5 (e-folded); the posterior distributions report informative measurements already at SNR∼6 for some noise realizations. 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. 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. Violations of the equation of state (EOS)-insensitive 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.

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Phys. Rev. D 112, 124001– Published 1 December, 2025
  • Received 23 May 2022
  • Revised 1 September 2025
  • Accepted 12 November 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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