Post-merger evolution of a neutron star-black hole binary with neutrino transport

TL;DR

This paper presents the first full general relativistic simulation of a black hole-neutron star merger with neutrino transport, revealing differences in neutrino emission, composition, and ejecta compared to simpler models, with implications for nucleosynthesis and electromagnetic signals.

Contribution

It introduces a novel implementation of neutrino transport in full GR simulations of black hole-neutron star mergers, providing new insights into post-merger disk evolution and ejecta properties.

Findings

Neutrino transport results in lower electron neutrino luminosity and less neutron-rich remnants.

A small amount of neutron-rich material is ejected during disk formation.

Neutrino effects influence nucleosynthesis and electromagnetic transient predictions.

Abstract

We present a first simulation of the post-merger evolution of a black hole-neutron star binary in full general relativity using an energy-integrated general relativistic truncated moment formalism for neutrino transport. We describe our implementation of the moment formalism and important tests of our code, before studying the formation phase of a disk after a black hole-neutron star merger. We use as initial data an existing general relativistic simulation of the merger of a neutron star of 1.4 solar mass with a black hole of 7 solar mass and dimensionless spin a/M=0.8. Comparing with a simpler leakage scheme for the treatment of the neutrinos, we find noticeable differences in the neutron to proton ratio in and around the disk, and in the neutrino luminosity. We find that the electron neutrino luminosity is much lower in the transport simulations, and that the remnant is less neutron-rich. The spatial distribution of the neutrinos is significantly affected by relativistic effects. Over the short timescale evolved, we do not observe purely neutrino-driven outflows. However, a small amount of material (3e-4Msun) is ejected in the polar region during the circularization of the disk. Most of that material is ejected early in the formation of the disk, and is fairly neutron rich. Through r-process nucleosynthesis, that material should produce high-opacity lanthanides in the polar region, and could thus affect the lightcurve of radioactively powered electromagnetic transients. We also show that by the end of the simulation, while the bulk of the disk is neutron-rich, its outer layers have a higher electron fraction. As that material would be the first to be unbound by disk outflows on longer timescales, the changes in Ye experienced during the formation of the disk could have an impact on the nucleosynthesis outputs from neutrino-driven and viscously-driven outflows. [Abridged]