We investigate the influence of magnetic fields on the evolution of binary neutron-star ( BNS ) merger remnants via three-dimensional ( 3D ) dynamical-spacetime general-relativistic ( GR ) magnetohydrodynamic ( MHD ) simulations .
We evolve a postmerger remnant with an initial poloidal magnetic field , resolve the magnetoturbulence driven by shear flows , and include a microphysical finite-temperature equation of state ( EOS ) .
A neutrino leakage scheme that captures the overall energetics and lepton number exchange is also included .
We find that turbulence induced by the magnetorotational instability ( MRI ) in the hypermassive neutron star ( HMNS ) amplifies magnetic field to beyond magnetar-strength ( 10 ^ { 15 } \mathrm { G } ) .
The ultra-strong toroidal field is able to launch a relativistic jet from the HMNS .
We also find a magnetized wind that ejects neutron-rich material with a rate of \dot { M } _ { \mathrm { ej } } \simeq 1 \times 10 ^ { -1 } \mathrm { M _ { \odot } s ^ { -1 } } .
The total ejecta mass in our simulation is 5 \times 10 ^ { -3 } \mathrm { M _ { \odot } } .
This makes the ejecta from the HMNS an important component in BNS mergers and a promising source of r -process elements that can power a kilonova .
The jet from the HMNS reaches a terminal Lorentz factor of \sim 5 in our highest-resolution simulation .
The formation of this jet is aided by neutrino-cooling preventing the accretion disk from protruding into the polar region .
As neutrino pair-annihilation and radiative processes in the jet ( which were not included in the simulations ) will boost the Lorentz factor in the jet further , our simulations demonstrate that magnetars formed in BNS mergers are a viable engine for short gamma-ray bursts ( sGRBs ) .