General-relativistic neutrino-radiation magnetohydrodynamic simulation of seconds-long black hole-neutron star mergers
Kota Hayashi, Sho Fujibayashi, Kenta Kiuchi, Koutarou Kyutoku, Yuichiro Sekiguchi, Masaru Shibata
Abstract
Seconds-long numerical-relativity simulations for black hole--neutron star mergers are performed for the first time to obtain a self-consistent picture of the merger and post-merger evolution processes. To investigate the case that tidal disruption takes place, we choose the initial mass of the black hole to be $5.4\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ or $8.1\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ with a dimensionless spin of 0.75. The neutron-star mass is fixed to be $1.35\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$. We find that after the tidal disruption, dynamical mass ejection takes place over $\ensuremath{\lesssim}10\text{ }\text{ }\mathrm{ms}$, together with the formation of a massive accretion disk. Subsequently, the magnetic field in the disk is amplified by the magnetic winding and magnetorotational instability, establishing a turbulent state and inducing angular momentum transport. The post-merger mass ejection by the magnetically induced viscous effect sets in at $\ensuremath{\sim}300--500\text{ }\text{ }\mathrm{ms}$ after the tidal disruption, at which the neutrino luminosity drops below $\ensuremath{\sim}{10}^{51.5}\text{ }\text{ }\mathrm{erg}/\mathrm{s}$, and continues for several hundred ms. A magnetosphere near the rotational axis of the black hole is developed after the matter and magnetic flux fall into the black hole from the accretion disk, and high-intensity Poynting flux generation sets in at a few hundred ms after the tidal disruption. The intensity of the Poynting flux becomes low after the significant post-merger mass ejection, because the opening angle of the magnetosphere increases. The lifetime of the stage with the strong Poynting flux is 1--2 s, which agrees with the typical duration of short-hard gamma-ray bursts.