  GW170817 and GRB 170817A provided direct evidence that binary neutron star
(NSNS) mergers can produce short gamma-ray bursts (sGRBs). However, questions
remain about the nature of the central engine. Depending on the mass, the
remnant from a NSNS merger may promptly collapse to a black hole (BH), form a
hypermassive neutron star (HMNS) which undergoes a delayed collapse to a BH, a
supramassive neutron star (SMNS) with a much longer lifetime, or an
indefinitely stable NS. There is strong evidence that a BH with an accretion
disk can launch a sGRB-compatible jet via the Blandford-Znajek mechanism, but
whether a supramassive star can do the same is less clear. We have performed
general relativistic magnetohydrodynamics (GRMHD) simulations of the merger of
both irrotational and spinning, equal-mass NSNSs constructed from a piecewise
polytropic representation of the SLy equation of state, with a range of
gravitational (ADM) masses that yield remnants with mass above and below the
supramassive limit. Each NS is endowed with a dipolar magnetic field extending
from the interior into the exterior, as in a radio pulsar. We examine cases
with different initial binary masses, including a case which produces a HMNS
which collapses to a BH, and lower mass binaries that produce SMNS remnants. We
find similar jet-like structures for both the SMNS and HMNS remnants that meet
our basic critera for an incipient jet. The outflow for the HMNS case is
consistent with a Blandford-Znajek (BZ) jet. There is sufficient evidence that
such BZ-powered outflows can break out and produce ulrarelativistic jets so
that we can describe the HMNS system as a sGRB progenitor. However, the
incipient jets from the SMNS remnants have much more baryon pollution and we
see indications of inefficient acceleration and mixing with the surrounding
debris. Therefore, we cannot conclude that SMNS outflows are the progenitors of
sGRBs.
