Relativistic Simulations of Accreting Neutron Stars

The physics of neutron-star magnetospheres interacting with accretion flows is dominated by a series of coupled nonlinear processes, requiring self-consistent multidimensional simulations for a quantitative interpretation of observations. These simulations have long been absent in the relativistic regime - vital for modeling accreting millisecond pulsars and other rapidly rotating systems - due to the challenge of coupling a magnetically dominated, nearly force-free magnetosphere with a magnetohydrodynamic accretion flow. Our group has recently developed a method that enables general-relativistic simulations of accreting, rotating neutron stars, which we have demonstrated with axisymmetric and preliminary 3D calculations. We propose to use this new method to perform an extensive set of 3D simulations, to investigate: (a) how neutron stars launch relativistic jets; (b) what determines the torque applied on the star, and how this relates to the observed cutoff of the pulsar spinfrequency distribution; (c) how electromagnetic winds interact with accretion flows beyond the pulsar's light cylinder, and whether this may resolve the puzzle of X-ray mode-switching in transitional millisecond pulsars; and (d) what can be learned about the general processes of accretion and jet launching by the first direct comparison of blackhole and neutron-star simulations. We expect this work to be broadly applicable to studies of all varieties of accreting neutron stars, from millisecond pulsars to pulsing ultraluminous X-ray sources, and to the closely related non-relativistic systems of accreting young stellar objects and T Tauri stars. This research is relevant to NASA's mission in space-borne high-energy astrophysics, as we aim to understand X-ray observations from Chandra, NuSTAR, and NICER.