On what date was the GW150914 black-hole merger signal detected?

GW150914 was detected on September 14, 2015. The signal was recorded by both Advanced LIGO interferometers at 09:50:45 UTC, during the detectors’ first observing run. Its name encodes the date: “GW” for gravitational wave and “150914” for 2015-09-14.

The event came from the merger of two black holes roughly 1.3 billion light-years away. Their final inspiral and collision produced a stronger gravitational-wave signal than either detector had previously observed. The waves changed the four-kilometre arm lengths by only about one-thousandth of a proton’s diameter, yet the matched signal appeared in both sites.

The detection was announced publicly on February 11, 2016, which is a common source of confusion. February 11 was the announcement date, not the detection date. GW150914 was the first direct detection of gravitational waves and the first direct evidence of a binary black-hole merger.