Impact of black hole spin on low-mass black hole-neutron star mergers

TL;DR

This study uses extensive simulations to explore how black hole spin influences ejecta and electromagnetic signals in low-mass black hole-neutron star mergers, revealing a new spin-driven ejecta mechanism.

Contribution

It is the most comprehensive analysis of spin effects in black hole-neutron star mergers, identifying spiral wave-driven ejecta and a spin-enhanced kilonova emission mechanism.

Findings

Fast ejecta velocities up to 0.6c for high spins

Ejecta mass increases with black hole spin

First identification of spiral wave-driven ejecta in these mergers

Abstract

The recent detection of GW230529 suggests that black hole-neutron star mergers may involve low-mass black holes, potentially producing detectable electromagnetic counterparts. Motivated by this, we perform eleven fully general-relativistic hydrodynamic simulations with and without neutrino treatment, targeting the inferred chirp mass of GW230529. We systematically vary the black hole spin from $a_{\mathrm{BH}} = 0.0$ to $0.8$ in steps of $0.1$, making this the most comprehensive study of spin effects in black hole-neutron star mergers to date. We confirm our earlier findings of fast-moving ejecta ($v \geq 0.6\,c$) in this parameter regime and demonstrate a clear spin dependence, with fast-ejecta masses reaching up to $\qty{\sim e-3}{\Mass\Sun}$ for $a_{\mathrm{BH}} = 0.8$. Most notably, we identify for the first time the presence of spiral wave-driven ejecta in black hole-neutron star mergers -- a phenomenon previously reported only in binary neutron star systems. The mass of this component grows significantly with spin, reaching levels up to $\qty{\sim 7e-3}{\Mass\Sun}$. These results establish a new spin-enhanced mechanism for powering blue kilonova emission in black hole-neutron star mergers, significantly extending the range of systems expected to produce observable electromagnetic counterparts.