Accretion-induced prompt black hole formation in asymmetric neutron star mergers, dynamical ejecta and kilonova signals
In 2003.06015 we present new numerical relativity results of neutron star mergers with chirp mass 1.188M⊙ and mass ratios q=1.67 and q=1.8 using finite-temperature equations of state (EOS), approximate neutrino transport and a subgrid model for magnetohydrodynamics-induced turbulent viscosity. The EOS are compatible with nuclear and astrophysical constraints and include a new microphysical model derived from ab-initio calculations based on the Brueckner-Hartree-Fock approach. We report for the first time evidence for accretion-induced prompt collapse in high-mass-ratio mergers, in which the tidal disruption of the companion and its accretion onto the primary star determine prompt black hole formation. As a result of the tidal disruption, an accretion disc of neutron-rich and cold matter forms with baryon masses ∼0.15M⊙, and it is significantly heavier than the remnant discs in equal-masses prompt collapse mergers. Massive dynamical ejecta of order ∼0.01M⊙ also originate from the tidal disruption. They are neutron rich and expand from the orbital plane with a crescent-like geometry. Consequently, bright, red and temporally extended kilonova emission is predicted from these mergers. Our results show that prompt black hole mergers can power bright electromagnetic counterparts for high-mass-ratio binaries, and that the binary mass ratio can be in principle constrained from multimessenger observations.