  By performing fully general relativistic magnetohydrodynamic simulations of
binary neutron star mergers, we investigate the possibility that the end result
of the merger is a stable magnetar. In particular, we show that, for a binary
composed of two equal-mass neutron stars (NSs) of gravitational mass M~1.2 Msun
and equation of state similar to Shen et al. at high densities, the merger
product is a stable NS. Such NS is found to be differentially rotating and
ultraspinning with spin parameter J/M^2~0.86, where J is its total angular
momentum, and it is surrounded by a disk of ~0.1 Msun. While in our global
simulations the magnetic field is amplified by about two orders of magnitude,
local simulations have shown that hydrodynamic instabilities and the onset of
the magnetorotational instability could further increase the magnetic field
strength up to magnetar levels. This leads to the interesting possibility that,
for some NS mergers, a stable and magnetized NS surrounded by an accretion disk
could be formed. We discuss the impact of these new results for the emission of
electromagnetic counterparts of gravitational wave signals and for the central
engine of short gamma-ray bursts.
