We present new numerical relativity results of neutron star mergers with chirp mass 1.188 M _ { \odot } 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 { \sim } 0.15 M _ { \odot } , and it is significantly heavier than the remnant discs in equal-masses prompt collapse mergers .
Massive dynamical ejecta of order { \sim } 0.01 M _ { \odot } 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 .