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We present the Dark Energy Camera (DECam) discovery of the opticalcounterpart of the first binary neutron star merger detected throughgravitational wave emission, GW170817. Our observations commenced 10.5 hourspost-merger, as soon as the localization region became accessible from Chile.We imaged 70 deg$^2$ in the $i$ and $z$ bands, covering 93\% of the initialintegrated localization probability, to a depth necessary to identify likelyoptical counterparts (e.g., a kilonova). At 11.4 hours post-merger we detecteda bright optical transient located $10.6''$ from the nucleus of NGC\,4993 atredshift $z=0.0098$, consistent (for $H_0 = 70$\, km s$^{-1}$ Mpc$^{-1}$) withthe distance of $40 \pm 8$\, Mpc reported by the LIGO Scientific Collaborationand the Virgo Collaboration (LVC). At detection the transient had magnitudes$i\approx 17.30$ and $z\approx 17.45$, and thus an absolute magnitude of $M_i =-15.7$, in the luminosity range expected for a kilonova. We identified 1,500potential transient candidates. Applying simple selection criteria aimed atrejecting background events such as supernovae, we find the transientassociated with NGC\,4993 as the only remaining plausible counterpart, andreject chance coincidence at the 99.5\% confidence level. We therefore concludethat the optical counterpart we have identified near NGC\,4993 is associatedwith GW170817. This discovery ushers in the era of multi-messenger astronomywith gravitational waves, and demonstrates the power of DECam to identify theoptical counterparts of gravitational-wave sources.

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. I. Dark Energy Camera Discovery of the Optical Counterpart