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Abstract

The merger of neutron stars drives a relativistic jet which can be observed
as a short gamma-ray burst. A strong large-scale magnetic field is necessary to
launch the relativistic jet. However, the magnetohydrodynamical mechanism to
build up this magnetic field remains uncertain. Here we show that the
$\alpha\Omega$ dynamo mechanism driven by the magnetorotational instability
builds up the large-scale magnetic field inside the long-lived binary neutron
star merger remnant by performing an {\it ab initio} super-high resolution
neutrino-radiation magnetohydrodynamics merger simulation in full general
relativity. As a result, the magnetic field induces the Poynting-flux dominated
relativistic outflow with the luminosity $\sim 10^{51}$\,erg/s and
magnetically-driven post-merger mass ejection with the mass $\sim 0.1M_\odot$.
Therefore, the magnetar scenario in binary neutron star mergers is possible.
These can be the engines of short-hard gamma-ray bursts and very bright
kilonovae. Therefore, this scenario is testable in future observation.