General-relativistic hydrodynamics simulation of a neutron star–sub-solar-mass black hole merger

Over the last few years, there has been an increasing interest in sub-solar-mass black holes due to their potential to provide valuable information about cosmology or the black hole population. Motivated by this, we study observable phenomena connected to the merger of a sub-solar-mass black hole with a neutron star. For this purpose, we perform new numerical-relativity simulations of a binary system composed of a black hole with mass 0.5M⊙ and a neutron star with mass 1.4M⊙. We investigate the merger dynamics of this exotic system and provide information about the connected gravitational-wave and kilonova signals. Our study indicates that current gravitational-waveform models cannot adequately describe such systems and that phenomenological relations connecting the binary parameters with the ejecta and remnant properties do not apply to our system. Furthermore, we find a dependence of the kilonova signal on the azimuthal viewing angle due to the asymmetric mass ejection. This first-of-its-kind simulation opens the door for studying sub-solar-mass black hole-neutron star mergers and could serve as a testing ground for future model development.