| Abstract: | On August 17, 2017 the merger of two compact objects with masses consistent
with two neutron stars was discovered through gravitational-wave (GW170817),
gamma-ray (GRB170817A), and optical (SSS17a/AT 2017gfo) observations. The
optical source was associated with the early-type galaxy NGC 4993 at a distance
of just $sim$40 Mpc, consistent with the gravitational-wave measurement, and
the merger was localized to be at a projected distance of $sim$2 kpc away from
the galaxy’s center. We use this minimal set of facts and the mass posteriors
of the two neutron stars to derive the first constraints on the progenitor of
GW170817 at the time of the second supernova (SN). We generate simulated
progenitor populations and follow the 3-dimensional kinematic evolution from
BNS birth to the merger time, accounting for pre-SN galactic motion, for
considerably different input distributions of the progenitor mass, pre-SN
semimajor axis, and SN kick velocity. Though not considerably tight, we find
these constraints to be comparable to those for Galactic BNS progenitors. The
derived constraints are very strongly influenced by the requirement of keeping
the binary bound after the second SN and having the merger occur relatively
close to the center of the galaxy. These constraints are insensitive to the
galaxy’s star-formation history, provided the stellar populations are older
than 1 Gyr. |
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