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

ABSTRACT The recent detection of GW230529 suggests that black hole–neutron star (BHNS) 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 BHNS mergers to date. We confirm our earlier findings of fast-moving ejecta ($v \ge 0.6\, c$) in this parameter regime and demonstrate a clear spin dependence, with fast-ejecta masses reaching up to ${\sim } 3 \times 10^{-4} \, \mathrm{M}_{\odot }$ for $a_{\mathrm{BH}} = 0.8$. Most notably, we identify for the first time the presence of spiral wave-driven ejecta in BHNS mergers – a phenomenon previously reported only in binary neutron star systems. The mass of this component grows significantly with spin, reaching levels up to $\sim 7 \times 10^{-3}\, \mathrm{M}_{\odot }$. These results establish a new spin-enhanced mechanism for powering blue kilonova emission in BHNS mergers, significantly extending the range of systems expected to produce observable electromagnetic counterparts.