Accretion-induced prompt black hole formation in asymmetric neutron star mergers, dynamical ejecta, and kilonova signals

Abstract

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.

Neutron star merger remnants

“…The prompt collapse models above indeed fail for large mass ratios q ∼ 1.5-2 [81]. In BNSs with increasing mass ratios and fixed chirp masses, the companion NS undergoes a progressively more significant tidal disruption.…”

“…Black holes produced by the collapse of irrotational binary merger remnants (either prompt collapse or short-lived) are found with dimensionless spin [74,80,81,90,96,97] 0.6 a BH 0.875 . (17) This interval can be expected from the merger quasiuniversal relations presented in Sect.…”

“…In high mass-ratio mergers with q 1.5 the companion NS is tidally disrupted and the disc is mainly formed by the tidal tail [81]. The latter is launched prior to merger and massive accretion discs are possible even if prompt BH formation occurs [34,81]. The angular momentum of these discs can be ∼60% larger than that of discs around BHs resulting from equal-mass mergers.…”

“…The accretion of discs around BHs formed in high-mass-ratio mergers is instead slower due to the larger disc's extension and angular momentum. Here, however, accretion is further driven by the fallback of the tidal tail that perturbs the disc inwards [81].…”

Binary Neutron Star Merger Simulations with a Calibrated Turbulence Model

“…Here, we calibrate a subgrid model using results from very-high-resolution GRMHD simulations performed by Kiuchi and collaborators [75], which were able to resolve all the unstable scales of the MRI for a binary system with an initially large magnetic field. We perform BNS merger simulations with microphysics and compare the results obtained with the newly calibrated turbulence model with those obtained using the prescription we proposed in [81], which was used in several other works [58,61,[87][88][89][90], and with those obtained with traditional GRHD simulations having no subgrid model. We find that turbulence can impact the postmerger evolution of BNSs in a quantitative way, and we discuss the implications for the interpretation of synthetic GW, EM, and nucleosynthesis yields from BNS merger simulations.…”