Listening to the long ringdown

A novel way to pinpoint the EOS in neutron-star cores

¹ Institut für Theoretische Physik, Goethe Universität, Max-von-Laue-Str. 1, 60438, Germany
² Department of Physics, The Ohio State University, Columbus, OH 43210, USA
³ Faculty of Science and Technology, University of Stavanger, Stavanger, 4036, Stavanger, Norway

^* e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Published online: 17 April 2026

Abstract

Gravitational waves (GWs) from binary neutron star (BNS) merger remnants complement constraints from the inspiral phase, mass-radius measurements, and microscopic theory by providing information about the neutron-star equation of state (EOS) at extreme densities. We perform general-relativistic simulations of BNS mergers using EOS models that span the uncertain high-density regime. We find a robust correlation between the ratio of energy and angular momentum lost during the late-time post-merger GW signal—the long ringdown—and the EOS at the highest densities in neutron star cores. Applying this correlation to post-merger GW signals reduces EOS uncertainty at several times saturation density, where no direct constraints currently exist.