- (1) University of California, Santa Cruz, grid.205975.c
- (2) Carnegie Observatories, grid.432988.c
- (3) University of Hawaii at Manoa, grid.410445.0
- (4) University of La Serena, grid.19208.32
- (5) Lawrence Berkeley National Laboratory, grid.184769.5
- (6) University of California, Berkeley, grid.47840.3f
- (7) Department of Astronomy and Astrophysics, The University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA.
- (8) University of Copenhagen, grid.5254.6, KU
- (9) Johns Hopkins University, grid.21107.35
- (10) Space Telescope Science Institute, grid.419446.a
On 17 August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo interferometer detected gravitational waves (GWs) emanating from a binary neutron star merger, GW170817. Nearly simultaneously, the Fermi and INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) telescopes detected a gamma-ray transient, GRB 170817A. At 10.9 hours after the GW trigger, we discovered a transient and fading optical source, Swope Supernova Survey 2017a (SSS17a), coincident with GW170817. SSS17a is located in NGC 4993, an S0 galaxy at a distance of 40 megaparsecs. The precise location of GW170817 provides an opportunity to probe the nature of these cataclysmic events by combining electromagnetic and GW observations.