# Numerical simulations of neutron star-black hole binaries in the near-equal-mass regime

## TL;DR

This paper presents numerical simulations of low-mass neutron star-black hole mergers, revealing smaller remnant disks and slower, neutron-rich ejecta, with implications for gravitational wave and electromagnetic signals similar to neutron star mergers.

## Contribution

It provides the first detailed simulations of low-mass NSBH mergers, including systems similar to GW170817, highlighting differences in ejecta and remnant properties compared to previous models.

## Key findings

- Remnant disks are less massive than previously thought.
- Ejecta are cold, neutron-rich, and slower than typical tidal disruption ejecta.
- Final black hole spin is higher than expected, up to 0.84.

## Abstract

Simulations of neutron star-black hole (NSBH) binaries generally consider black holes with masses in the range $(5-10)M_\odot$, where we expect to find most stellar mass black holes. The existence of lower mass black holes, however, cannot be theoretically ruled out. Low-mass black holes in binary systems with a neutron star companion could mimic neutron star-neutron (NSNS) binaries, as they power similar gravitational wave (GW) and electromagnetic (EM) signals. To understand the differences and similarities between NSNS mergers and low-mass NSBH mergers, numerical simulations are required. Here, we perform a set of simulations of low-mass NSBH mergers, including systems compatible with GW170817. Our simulations use a composition and temperature dependent equation of state (DD2) and approximate neutrino transport, but no magnetic fields. We find that low-mass NSBH mergers produce remnant disks significantly less massive than previously expected, and consistent with the post-merger outflow mass inferred from GW170817 for moderately asymmetric mass ratio. The dynamical ejecta produced by systems compatible with GW170817 is negligible except if the mass ratio and black hole spin are at the edge of the allowed parameter space. That dynamical ejecta is cold, neutron-rich, and surprisingly slow for ejecta produced during the tidal disruption of a neutron star : $v\sim (0.1-0.15)c$. We also find that the final mass of the remnant black hole is consistent with existing analytical predictions, while the final spin of that black hole is noticeably larger than expected -- up to $\chi_{\rm BH}=0.84$ for our equal mass case.