GW151226

GW151226 is the designation given to a gravitational-wave signal detected on December 26, 2015, by the Laser Interferometer Gravitational-Wave Observatory (LIGO). It was the second confirmed gravitational-wave event and the first attributed to a binary black hole merger with masses significantly lower than those in the first detection, GW150914. The event marked an important expansion of the observed black-hole population and the scope of gravitational-wave astronomy.

The signal originated from the coalescence of two black holes with masses approximately 14 and 7 solar masses. The merger produced a final black hole of about 20 solar masses, with roughly three solar masses converted into energy as gravitational waves. The event occurred at a luminosity distance of roughly 440 megaparsecs, corresponding to about 1.4 billion light-years from Earth. The spins of the black holes were modest, and the signal matched predictions of general relativity for a black-hole merger.

GW151226 was observed by both LIGO detectors in Hanford, Washington, and Livingston, Louisiana. The Virgo detector did not detect it, but aided localization when possible. The network information constrained the source to a sky region spanning thousands of square degrees. No confirmed electromagnetic counterpart has been associated with GW151226, despite targeted follow-up efforts across the spectrum. A transient gamma-ray signal reported by other instruments shortly after the event was not definitively linked to the merger.

The detection reinforced the gravitational-wave paradigm, contributed to tests of general relativity in the strong-field regime, and helped establish the growing field of multi-messenger astronomy through subsequent observations of binary mergers.