The interplay of disk wind and dynamical ejecta in the aftermath of neutron star - black hole mergers

TL;DR

This study investigates how disk winds and dynamical ejecta interact after neutron star-black hole mergers, revealing that winds can suppress fallback accretion and contribute to late-time gamma-ray burst activity, with minimal impact from radioactive heating.

Contribution

It introduces a combined two- and three-dimensional hydrodynamic simulation approach to study the interaction of ejecta and disk winds post-merger, highlighting their effects on fallback and potential gamma-ray burst engines.

Findings

Disk wind suppresses fallback accretion after ~100 ms.

Late-time disk accretion follows a t^{-2.2} power-law.

Radioactive heating does not significantly alter fallback or wind properties.

Abstract

We explore the evolution of the different ejecta components generated during the merger of a neutron star (NS) and a black hole (BH). Our focus is the interplay between material ejected dynamically during the merger, and the wind launched on a viscous timescale by the remnant accretion disk. These components are expected to contribute to an electromagnetic transient and to produce r-process elements, each with a different signature when considered separately. Here we introduce a two-step approach to investigate their combined evolution, using two- and three-dimensional hydrodynamic simulations. Starting from the output of a merger simulation, we identify each component in the initial condition based on its phase space distribution, and evolve the accretion disk in axisymmetry. The wind blown from this disk is injected into a three-dimensional computational domain where the dynamical ejecta is evolved. We find that the wind can suppress fallback accretion on timescales longer than ~100 ms. Due to self-similar viscous evolution, the disk accretion at late times nevertheless approaches a power-law time dependence $\propto t^{-2.2}$. This can power some late-time GRB engine activity, although the available energy is significantly less than in traditional fallback models. Inclusion of radioactive heating due to the r-process does not significantly affect the fallback accretion rate or the disk wind. We do not find any significant modification to the wind properties at large radius due to interaction with the dynamical ejecta. This is a consequence of the different expansion velocities of the two components.