The Collimation of Relativistic Jets in Post-Neutron Star Binary Merger Simulations

The gravitational waves from the binary neutron star merger GW170817 were accompanied by a multiwavelength electromagnetic counterpart, which confirms the association of the merger with a short gamma-ray burst (sGRB). The afterglow observations implied that the event was accompanied by a narrow, ∼5°, and powerful, ∼10⁵⁰ erg, jet. We study the propagation of a Poynting flux-dominated jet within the merger ejecta (kinematic, neutrino-driven, and magnetorotational instability turbulence-driven) of a neutrino-radiation-GRMHD simulation of two coalescing neutron stars. We find that the presence of a postmerger low-density/low-pressure polar cavity, which arose due to angular momentum conservation, is crucial to let the jet break out. At the same time the ejecta collimates the jet to a narrow opening angle. The collimated jet has a narrow opening angle of ∼4°-7° and an energy of 10⁴⁹-10⁵⁰ erg, in line with the observations of GW170817 and other sGRBs.