Listening to the long ringdown

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.