Neutron Stars: to Jet or not to Jet?

Both accreting neutron stars and black holes produce relativistic jets powered by large-scale magnetic field. However, only neutron stars can possess the magnetic field of their own. Neutron star accretion systems can therefore come in two polarities: (i) the “parallel” polarity where the neutron star magnetic field is aligned with that supplied by the accretion disk and (ii) the “anti-parallel” polarity where the stellar and disk fields are anti-aligned. How does the extra degree of freedom - the magnetic polarity - affect the ability of neutron stars to produce relativistic jets? We address this question by carrying out a suite of non-radiative 3D general relativistic magnetohydrodynamic (GRMHD) simulations of accreting neutron stars. We find that for systems with strong stellar magnetic fields, when the neutron star magnetosphere truncates the accretion disk at large distances, where the disk rotates at the same or slower speed than the magnetosphere, both neutron star polarities produce jets of similar power. However, for weaker magnetic fields, when the disk rotates faster than the magnetosphere, jet formation by neutron stars of the parallel polarity is suppressed. We discuss the physical reasons for this suppression and its astrophysical implications.