Propagation of a Realistic Magnetar Jet through a Binary Neutron Star Merger Medium and Implications for Short Gamma-Ray Bursts

Abstract

The origin of short gamma-ray bursts is associated with outflows powered by the remnant of a binary neutron star merger. This remnant can be either a black hole or a highly magnetized, fast-spinning neutron star, also known as a magnetar. Here we present the results of two relativistic magnetohydrodynamical simulations aimed at investigating the large-scale dynamics and propagation of magnetar collimated outflows through the medium surrounding the remnant. The first simulation evolves a realistic jet by injecting external simulation data, while the second evolves an analytical model jet with similar properties for comparison. We find that both outflows remain collimated and successfully emerge through the static medium surrounding the remnant. However, they fail to attain relativistic velocities and only reach a mean maximum speed of ∼0.7c for the realistic jet and ∼0.6c for the analytical jet. We also find that the realistic jet has a much more complex structure. The lack of highly relativistic speeds, which makes these jets unsuitable as short gamma-ray burst sources, is due to numerical limitations and is not general to all possible magnetar outflows. A jet like the one we study, however, could give rise to or augment a blue kilonova component. In addition, it would make the propagation of a relativistic jet easier, should one be launched after the neutron star collapses into a black hole.