Abstract
We present the Dark Energy Camera (DECam) discovery of the optical counterpart of the first binary neutron star
merger detected through gravitational-wave emission, GW170817. Our observations commenced 10.5 hr post-merger,
as soon as the localization region became accessible from Chile. We imaged 70 deg2 in the i and z bands, covering
93% of the initial integrated localization probability, to a depth necessary to identify likely optical counterparts
(e.g., a kilonova). At 11.4 hr post-merger we detected a bright optical transient located 10. 6 from the nucleus of
NGC 4993 at redshift z = 0.0098, consistent (for H0 = 70 km s−1 Mpc−1
) with the distance of 40 ± 8 Mpc reported
by the LIGO Scientific Collaboration and the Virgo Collaboration (LVC). At detection the transient had magnitudes
of i = 17.3 and z = 17.4, and thus an absolute magnitude of Mi = -15.7, in the luminosity range expected for a
kilonova. We identified 1500 potential transient candidates. Applying simple selection criteria aimed at rejecting
background events such as supernovae, we find the transient associated with NGC 4993 as the only remaining
plausible counterpart, and reject chance coincidence at the 99.5% confidence level. We therefore conclude that the
optical counterpart we have identified near NGC 4993 is associated with GW170817. This discovery ushers in the era
of multi-messenger astronomy with gravitational waves and demonstrates the power of DECam to identify the optical
counterparts of gravitational-wave sources.