Dynamical ejecta from precessing neutron star-black hole mergers with a hot, nuclear-theory based equation of state

TL;DR

This paper presents simulations of precessing neutron star-black hole mergers using a nuclear-theory based equation of state, analyzing the effects on ejecta properties and improving numerical methods.

Contribution

It introduces the first simulations of precessing NS-BH mergers with a realistic, hot, nuclear-theory based equation of state and an improved mesh-refinement algorithm.

Findings

Remnant and ejecta masses are consistent with simpler models.

Ejecta velocity differences are explained by composition assumptions.

Updated velocity estimates account for low-density, cold material effects.

Abstract

Neutron star-black hole binaries are among the strongest sources of gravitational waves detectable by current observatories. They can also power bright electromagnetic signals (gamma-ray bursts, kilonovae), and may be a significant source of production of r-process nuclei. A misalignment of the black hole spin with respect to the orbital angular momentum leads to precession of that spin and of the orbital plane, and has a significant effect on the properties of the post-merger remnant and of the material ejected by the merger. We present a first set of simulations of precessing neutron star-black hole mergers using a hot, composition dependent, nuclear-theory based equation of state (DD2). We show that the mass of the remnant and of the dynamical ejecta are broadly consistent with the result of simulations using simpler equations of state, while differences arise when considering the dynamics of the merger and the velocity of the ejecta. We show that the latter can easily be understood from assumptions about the composition of low-density, cold material in the different equations of state, and propose an updated estimate for the ejecta velocity which takes those effects into account. We also present an updated mesh-refinement algorithm which allows us to improve the numerical resolution used to evolve neutron star-black hole mergers.