Recoil Velocity of Binary Neutron Star Remnants

The LIGO-Virgo gravitational wave detectors have 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 post-merger kilonova. In this work, we combine gravitational radiation as well as dynamical ejecta distributions, computed by the Computational Relativity (CoRe) numerical simulations, to get accurate estimates for BNS remnant recoil velocities. Based on this, we also obtain a numerical relativity fit for recoil velocity as a function of BNS parameters. BNS remnant recoil velocities play an important role in determining if the remnant is retained by its environment for future hierarchical mergers in a dynamical formation scenario which, in turn, can populate black holes within the lower mass gap of $\sim 3-5 M_{\odot}$.