Counterpart of GW170817. I. DECam obs.

Our observations commenced 10.5hr post-merger, as soon as the localization region became accessible from Chile. We imaged 70deg^2^ 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 11.4hr post-merger we detected a bright optical transient located 10.6" from the nucleus of NGC 4993 at redshift z=0.0098, consistent (for H_0_=70km/s/Mpc) with the distance of 40+/-8Mpc 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.