General-relativistic neutrino-radiation magnetohydrodynamics simulation of seconds-long black hole-neutron star mergers: Dependence on initial magnetic field strength, configuration, and neutron-star equation of state

Abstract

Numerical-relativity simulations for seconds-long black hole-neutron starmergers are performed to obtain a self-consistent picture starting from theinspiral and the merger throughout the post-merger stages for a variety ofsetups. Irrespective of the initial and computational setups, we findqualitatively universal evolution processes: The dynamical mass ejection takesplace together with a massive accretion disk formation after the neutron staris tidally disrupted; Subsequently, the magnetic field in the accretion disk isamplified by the magnetic winding, Kelvin-Helmholtz instability, andmagnetorotational instability, which establish a turbulent state inducing thedynamo and angular momentum transport; The post-merger mass ejection by theeffective viscous effects stemming from the magnetohydrodynamics turbulencesets in at $\sim300$-$500$ ms after the merger and continues for severalhundred ms; A magnetosphere near the black-hole spin axis is developed and thecollimated strong Poynting flux is generated with its lifetime of $\sim0.5$-$2$s. The model of no equatorial-plane symmetry shows the reverse of themagnetic-field polarity in the magnetosphere, which is caused by the dynamoassociated with the magnetorotational instability in the accretion disk. Themodel with initially toroidal fields shows the tilt of the disk andmagnetosphere in the late post-merger stage because of the anisotropicpost-merger mass ejection. These effects could terminate the strongPoynting-luminosity stage within the timescale of $\sim0.5$-$2$ s.<br