The distant binary of two super massive black holes, dubbed QJ287, flared within four hours of its predicted time in July 2019, proving the existence of this system.

The central black hole has a mass 18 billion times that of the Sun. The smaller black hole has a mass of 150 million Suns. Its orbit is twelve years long, and when it makes its close approach the interaction between these two monsters causes high energy flares.

We know all this because astronomers have been watching OJ 287 since the 1890s, before they knew what it was. In the intervening century, the system has shot off two outbursts roughly every 12 years, almost like clockwork.

Yet this pattern took time to decipher, as the bigger black hole in OJ 287 is also a blazar. Its black hole, or the disk that feeds it, powers twin plasma jets shooting out along opposite directions, and one of these jets is pointed almost right at Earth. The volatility of this plasma-and-photon stream makes OJ 287 a highly variable visible-light source. It wasn’t until a century after its discovery that astronomers realized that there was a periodic signal hidden within the noise — and that dual dancing black holes could cause it.

Observations in 2005 confirmed those ideas, and astronomers made increasingly precise predictions for subsequent flares in 2007 and 2015. Now, Seppo Laine (Spitzer Science Center, Caltech), Lankeswar Dey (Tata Institute of Fundamental Research, India), and colleagues are publishing observations of the latest flare in the Astrophysical Journal. The authors predicted, and then watched for, a flare expected to arrive in the early hours of July 31, 2019.

QJ287 is 3.5 billion light years away, which makes this prediction and the observations even more remarkable.