Figure 1. (Adapted from figure 1 of our publication). The gravitational wave event GW151226 as observed by the twin Advanced LIGO instruments: LIGO Hanford (left) and LIGO Livingston (right). The images show the data recorded by the detectors during the last second before merger as the signal varies as a function of time (in seconds) and frequency (in Hertz or the number of wave cycles per second). To be certain that a real gravitational wave has been observed, we compare the data from the detectors against a pre-defined set of models for merging binaries. This allows us to find gravitational wave signals which are buried deep in the noise from the instruments and nearly impossible to find by eye. The animation shows the detector data with and without removing the best-matching model gravitational-wave signal, making it much easier to identify. The signal can be seen sweeping up in frequency as the two black holes spiral together. This signal is much more difficult to spot by eye than the first detection GW150914!