Relativistic simulations of black hole-neutron star coalescence: the jet emerges

Abstract: We perform magnetohydrodynamic simulations in full general relativity (GRMHD) of a binary black hole-neutron star on a quasicircular orbit that undergoes merger. The binary mass ratio is 3:1, the black hole initial spin parameter $a/m=0.75$ ($m$ is the black hole Christodoulou mass) aligned with the orbital angular momentum, and the neutron star is an irrotational $\Gamma=2$ polytrope. About two orbits prior to merger (at time $t=t_B$), we seed the neutron star with a dynamically weak interior dipole magnetic field that extends into the stellar exterior. At $t=t_B$ the exterior has a low-density atmosphere with constant plasma parameter $\beta\equiv P_{\rm gas}/P_{\rm mag}$. Varying $\beta$ at $t_B$ in the exterior from $0.1$ to $0.01$, we find that at a time $\sim 4000{\rm M} \sim 100(M_{\rm NS}/1.4M_\odot)$ms [M is the total (ADM) mass] following the onset of accretion of tidally disrupted debris, magnetic winding above the remnant black hole poles builds up the magnetic field sufficiently to launch a mildly relativistic, collimated outflow - an incipient jet. The duration of the accretion and the lifetime of the jet is $\Delta t\sim 0.5(M_{\rm NS}/1.4M_\odot)$s. Our simulations furnish the first explicit examples in GRMHD which show that a jet can emerge following a black hole - neutron star merger.