Gravitational scattering of two neutron stars

TL;DR

This paper presents the first numerical relativity simulations of neutron star scattering, exploring strong-field dynamics, matter effects, and gravitational waveforms, with implications for understanding tidal interactions in relativistic two-body systems.

Contribution

First numerical relativity simulations of neutron star scattering, analyzing dynamics, matter effects, and gravitational waveforms, and comparing with theoretical models.

Findings

Quantitative agreement with effective-one-body predictions at large angular momenta

Significant discrepancies in tidal contributions near capture threshold

Matter effects imprint on waveforms during close encounters

Abstract

We present the first numerical relativity simulations of the gravitational scattering of two neutron stars. Constraint-satisfying initial data for two equal-mass nonspinning sequences are constructed at fixed energy and various initial angular momenta (impact parameter) and evolved with Einstein equations through the scattering process. The strong-field scattering dynamics are explored up to scattering angles of $220^\circ$ and the threshold of dynamical captures. The transition to bound orbits is aided by significant mass ejecta up to baryon mass ${\sim}0.1M_\odot$. A quantitative comparison with predictions of the scattering angle from state-of-the-art effective-one-body and post-Minkowskian calculations indicates quantitative agreement for large initial angular momenta although significant discrepancies in the tidal contribution emerge toward the capture threshold. Gravitational waveforms and radiated energy are in qualitative agreement with the analogous black hole problem and state-of-the-art effective-one-body predictions. Toward the capture threshold waveforms from scattering dynamics carry a strong imprint of matter effects, including the stars' $f$-mode excitations during the close encounter. Overall, our simulations open a new avenue to study tidal interactions in the relativistic two-body problem.