  The origin of short gamma-ray bursts (sGRBs) 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, fastly spinning neutron star, also
known as a magnetar. Here, we present the results of two relativistic
magnetohydrodynamical (RMHD) 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, that makes these jets
unsuitable as short GRB sources, is due to numerical limitations and 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.
