The First Unambiguous Electromagnetic Counterpart to a Gravitational-Wave Signal: GRB 170817A and GW170817

On 2017 August 17 at 12:41:06 UTC the Fermi Gamma-ray Burst Monitor (GBM) detected and triggered on the short gamma-ray burst (GRB) 170817A. Two seconds prior LIGO triggered on a binary compact merger candidate, the first from the merging for two neutron stars, associated with the GRB. This is the first unambiguous joint detection of an event in both electromagnetic and gravitational waves, marking the beginning of gravitational-wave multimessenger astronomy. We discuss the GBM observations and analysis of GRB 170817A and the science enabled by the joint GW-GRB detection.