Relativistic jets from millisecond proto-magnetars

Rapidly rotating, strongly magnetized neutron stars (``millisecond
proto-magnetars'') 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 (GRB) 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 proto-magnetars
can launch ultra-relativistic jets at early times, as needed to power
short-duration GRB.  We present three-dimensional general-relativistic
magnetohydrodynamic simulations of neutrino-heated proto-magnetar
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 $P \approx 1\,\mathrm{ms}$, centrifugal forces strongly
enhance mass loss near the rotational equator, producing a dense,
sub-relativistic outflow ($v \sim 0.1c$). 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 $\sigma
\sim 30-100$, sufficient to reach bulk Lorentz factors
$\Gamma_{\infty} \sim 100$ on larger scales.  The resulting angular
stratification of the outflow energy into ultra-relativistic polar and
sub-relativistic equatorial components is broadly consistent with the
observed partition between beaming-corrected GRB energies and
supernova/kilonova ejecta. Our results demonstrate that millisecond
proto-magnetars can launch relativistic jets within seconds of
formation and highlight their potential role in powering the diverse
electromagnetic counterparts of compact-object explosions.