  On August 17, 2017 at 12:41:04 UTC the Advanced LIGO and Advanced Virgo
gravitational-wave detectors made their first observation of a binary neutron
star inspiral. The signal, GW170817, was detected with a combined
signal-to-noise ratio of 32.4 and a false-alarm-rate estimate of less than one
per $8.0\times10^4$ years. We infer the component masses of the binary to be
between 0.86 and 2.26 $M_\odot$, in agreement with masses of known neutron
stars. Restricting the component spins to the range inferred in binary neutron
stars, we find the component masses to be in the range 1.17 to 1.60 $M_\odot$,
with the total mass of the system $2.74^{+0.04}_{-0.01}\,M_\odot$. The source
was localized within a sky region of 28 deg$^2$ (90% probability) and had a
luminosity distance of $40^{+8}_{-14}$ Mpc, the closest and most precisely
localized gravitational-wave signal yet. The association with the gamma-ray
burst GRB 170817A, detected by Fermi-GBM 1.7 s after the coalescence,
corroborates the hypothesis of a neutron star merger and provides the first
direct evidence of a link between these mergers and short gamma-ray bursts.
Subsequent identification of transient counterparts across the electromagnetic
spectrum in the same location further supports the interpretation of this event
as a neutron star merger. This unprecedented joint gravitational and
electromagnetic observation provides insight into astrophysics, dense matter,
gravitation and cosmology.
