Relativistic Jets from Millisecond Protomagnetars

Abstract Rapidly rotating, strongly magnetized neutron stars (“millisecond protomagnetars”) formed in stellar core collapse, neutron star mergers, and white dwarf accretion-induced collapse have long been proposed as central engines of gamma-ray bursts (GRBs) and accompanying supernovae/kilonovae. However, during the first few seconds after birth, neutrino heating drives baryon-rich winds from the neutron star surface, potentially limiting the magnetization and achievable Lorentz factors of the outflow and casting doubt on whether protomagnetars can launch ultrarelativistic jets at early times, as needed to power short-duration GRBs. We present three-dimensional general-relativistic magnetohydrodynamic simulations of neutrino-heated protomagnetar winds that incorporate M0 neutrino transport. While the global wind properties broadly agree with previous analytic estimates calibrated to one-dimensional models, our simulations reveal essential multidimensional effects. For rapidly rotating models with spin periods P ≈ 1 ms, centrifugal forces strongly enhance mass loss near the rotational equator, producing a dense, subrelativistic outflow ( v ∼ 0.1 c ). This equatorial wind naturally confines and collimates less baryon-loaded outflows emerging from higher latitudes, leading to the formation of a structured bipolar jet with a peak magnetization along the pole up to σ ∼ 30–100, sufficient to reach bulk Lorentz factors Γ ∞ ∼ 100 on larger scales. The resulting angular stratification of the outflow energy into ultrarelativistic polar and subrelativistic equatorial components is broadly consistent with the observed partition between beaming-corrected GRB energies and supernova/kilonova ejecta. Our results demonstrate that millisecond protomagnetars can launch relativistic jets within seconds of formation and highlight their potential role in powering the diverse electromagnetic counterparts of compact-object explosions.