General-relativistic neutrino-radiation magnetohydrodynamic simulation of seconds-long black hole-neutron star mergers

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 Mo or 8.1 Mo with a dimensionless spin of 0.75. The neutron-star mass is fixed to be 1.35 Mo. We find that after the tidal disruption, dynamical mass ejection takes place over less than or similar to 10 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 -300-500 ms after the tidal disruption, at which the neutrino luminosity drops below -1051.5 erg=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.