170817A

170817A, designated GRB 170817A, is a short gamma-ray burst that was detected on August 17, 2017. It was observed by the Fermi Gamma-ray Burst Monitor and INTEGRAL, and it occurred in temporal association with the gravitational-wave event GW170817, produced by a merger of two neutron stars. The source is located in the lenticular galaxy NGC 4993, at a distance of about 40 megaparsecs (roughly 130 million light-years). The gamma-ray signal arrived about 1.7 seconds after the gravitational waves.

The gamma-ray emission from GRB 170817A was unusually weak for a short GRB, with an isotropic-equivalent energy on the order of 10^46 erg. Follow-up observations across the electromagnetic spectrum identified a bright optical and near-infrared counterpart, later designated AT 2017gfo, consistent with a kilonova powered by the radioactive decay of heavy elements formed through rapid neutron capture (r-process). X-ray and radio emissions were detected in subsequent days to weeks, providing a broad multi-wavelength picture of the event.

Significance and impact: GRB 170817A was the first confirmed joint detection of gravitational waves and electromagnetic radiation from a neutron-star merger, marking the birth of multi-messenger astronomy. The event provided direct evidence linking short gamma-ray bursts to neutron-star mergers, offered insights into jet structure and viewing angle, and supported models of kilonovae as sites of heavy-element nucleosynthesis. It also enabled independent cosmological measurements through the standard siren approach and spurred advances in coordinating rapid, multi-instrument follow-up observations.