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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 M_ or 8. 1 M_ with a dimensionless spin of 0. 75. The neutron-star mass is fixed to be 1. 35 M_. We find that after the tidal disruption, dynamical mass ejection takes place over 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 10^51. 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.
Hayashi et al. (Fri,) studied this question.