  11 hours after the detection of gravitational wave source GW170817 by the
Laser Interferometer Gravitational-Wave Observatory and Virgo Interferometers,
an associated optical transient SSS17a was discovered in the galaxy NGC 4993.
While the gravitational wave data indicate GW170817 is consistent with the
merger of two compact objects, the electromagnetic observations provide
independent constraints of the nature of that system. Here we synthesize all
optical and near-infrared photometry and spectroscopy of SSS17a collected by
the One-Meter Two-Hemisphere collaboration. We find that SSS17a is unlike other
known transients. The source is best described by theoretical models of a
kilonova consisting of radioactive elements produced by rapid neutron capture
(the r-process). We find that SSS17a was the result of a binary neutron star
merger, reinforcing the gravitational wave result.
