Figure 1: Artist’s impression of a black hole.
Gravitational waves are ripples propagating through the fabric of spacetime. Abbott et al. (2016) present the observation of a gravitational wave signal designated as GW151226. This is the second direct detection of a gravitational wave signal and it was observed by two detectors of the Laser Interferometer Gravitational Wave Observatory (LIGO) on 26 December 2015 at 03:38:53 UTC. The signal has significance greater than 5σ, and it was observable for approximately one second. The gravitational waves first hit the LIGO observatory in Livingston, Louisiana. 1.1 milliseconds later, they passed through the LIGO observatory in Hanford, Washington.
GW151226 was produced from the coalescence of two stellar-mass black holes, and the observed gravitational wave signal originated from the final stages of the two black holes spiralling into each other. The primary and secondary black holes have ~14.2 and ~7.5 times the mass of the Sun, respectively. After merger, the final black hole has ~20.8 times the mass of the Sun. The final mass is lower than the sum of the initial masses because approximately one solar-mass was radiated away as pure energy, in the form of gravitational waves. This is because it takes energy to distort spacetime and create gravitational waves. GW151226 is estimated to have occurred at a distance of approximately 1.4 billion light years away.
Figure 2: Gravitational signal from GW151226. Abbott et al. (2016)
LIGO can only detect gravitational waves generated from massive objects orbiting each other faster than ~30 times per second. GW151226 was observed for a full second. For comparison, the gravitational wave signal from the first binary black hole merger detected by LIGO, designated GW150914, was observable for only about 0.2 seconds, just 1/5 the duration of GW151226. This is because the black holes for GW150914 were bigger than the ones that generated GW151226, and bigger black holes run into each other before they can orbit around one another fast enough to be detectable for a longer period of time by LIGO. Smaller black holes, like the ones that generated GW151226, can get much closer together before they merge, allowing them to orbit sufficiently fast around one another to be detectable by LIGO over a longer period of time.
Reference:
Abbott et al. (2016), "GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence", Phys. Rev. Lett. 116, 241103