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

We perform magnetohydrodynamic simulations in full general relativity (GRMHD) of a binary black hole-neutron star (BHNS) 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 γ = 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 a constant plasma parameter β ≡ P_gas/P_mag. Varying β at t_B in the exterior from 0.1 to 0.01, we find that at a time ∼4000M ∼ 100(M_NS/1.4M_⊙)(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 δt ∼ 0.5(M_NS/1.4M_⊙)s. Our simulations furnish the first explicit examples in GRMHD that show that a jet can emerge following a BHNS merger.