Recoil Velocity of Binary Neutron Star Merger Remnants

TL;DR

This paper estimates the recoil velocities of binary neutron star merger remnants by combining gravitational wave data and dynamical ejecta simulations, highlighting the dominant role of matter ejection in imparting recoil.

Contribution

It introduces a method to accurately estimate BNS remnant recoil velocities by integrating gravitational radiation and matter ejection data from numerical simulations, advancing understanding of post-merger dynamics.

Findings

Matter ejection dominates recoil velocities over gravitational wave emission.

Recoil velocities influence the retention of remnants for future mergers.

Accurate estimates can inform the formation of black hole binaries in the lower mass gap.

Abstract

The LIGO-Virgo gravitational wave detectors have confidently observed 4 events involving neutron stars: two binary neutron star (BNS) mergers (GW170817 and GW190425), and two neutron star-black hole mergers (GW200105 and GW200115). However, our theoretical understanding of the remnant properties of such systems is incomplete due to the complexities related to the modeling of matter effects and the very high computational cost of corresponding numerical relativity simulations. An important such property is the recoil velocity, which is imparted onto the remnant due to the anisotropic emission of gravitational radiation and the dynamical ejection of matter in the kilonova. In this work, we combine gravitational radiation as well as dynamical ejecta distributions, computed by the Computational Relativity numerical simulations, to get accurate estimates for BNS remnant recoil velocities. We find that recoils due to ejection of matter dominate those caused by gravitational wave emission. Knowledge of BNS remnant recoil velocities is important in determining if the remnant is retained by its environment for future hierarchical mergers which, in turn, can form binaries with black holes in the so-called lower mass gap of 3 to 5 solar masses.