The first gravitational wave event detected by LIGO was GW150914. The signal reached the LIGO observatories in Hanford, Washington, and Livingston, Louisiana, at 09:50:45 UTC on September 14, 2015. Its name combines “GW” for gravitational wave with the date of detection: 150914.
GW150914 came from the merger of two black holes roughly 1.3 billion light-years away. Their final inspiral produced a rapidly rising “chirp” lasting about 0.2 seconds, as the signal’s frequency climbed from roughly 35 hertz to 250 hertz. The merged remnant had a mass of about 62 Suns, with approximately three solar masses converted into gravitational-wave energy.
LIGO announced the discovery on February 11, 2016. It was the first direct detection of gravitational waves, confirming a major prediction of general relativity and the first observed merger of two stellar-mass black holes. GW151226 was another LIGO event detected later, while GW170817 was the first gravitational-wave event observed from merging neutron stars and accompanied by electromagnetic signals.