At the center of the galaxy OJ 287, lies one of the most massive black holes known, a colossal object weighing 18 billion times the mass of the Sun. It's surrounded by a disk of hot material and orbited by another, smaller black hole. This one is 150 million solar masses, lightweight in comparison, but still over 30 times the mass of Sagittarius A*, the black hole located at the heart of the Milky Way.
This unusual setup produces a distinct signature flare. Every time the smaller black hole passes through the disk of material surrounding the larger, a flare of energy brighter than a trillion stars combined is released. This happens twice every 12 years, but at irregular intervals due to the complex interaction between the massive objects.
Attempting to model the two bodies to estimate when a flare may occur has been a challenge for astronomers. One model in 2010 managed to predict a flare in December 2015 to within three weeks. In 2018, astronomers in India, led by Lankeswar Dey, a graduate student at the Tata Institute of Fundamental Research in Mumbai, produced a model that predicted a flare would occur on July 31, 2019, with an uncertainty of just four hours. Now, as reported in The Astrophysical Journal Letters, those researchers have described observing it, confirming their model is correct.
The observations were only possible thanks to NASA’s Spitzer observatory, the infrared space telescope that was retired early this year, and the reason is actually quite simple. OJ 287 was actually behind the Sun from the Earth’s position, but as Spitzer was in a trailing orbit, far away from Earth, it was in the right place at the right time to catch the flare.
"When I first checked the visibility of OJ 287, I was shocked to find that it became visible to Spitzer right on the day when the next flare was predicted to occur," lead author Seppo Laine, an associate staff scientist at Caltech/IPAC, who oversaw Spitzer's observations of the system, said in a statement. "It was extremely fortunate that we would be able to capture the peak of this flare with Spitzer because no other human-made instruments were capable of achieving this feat at that specific point in time."
The accurate predictions were down to a deep understanding of gravity and black holes. These two massive objects release energy as gravitational waves as they orbit each other. The larger the object's mass, the larger the gravitational waves generated, and it's though the waves from the larger black hole here are so energetic that they alter the orbit of the smaller black hole. The inclusion of this into the model reduced the window of when a flare may occur to just 36 hours.
The final nine-fold improvement was due to the inclusion of the “no-hair theorem”, the idea that a black hole's properties boil down to three classical parameters: mass, electric charge, and angular momentum, and all the other information is not necessary. The orbit of the smaller black hole is influenced only by the mass of the larger one, and this mass is uniformly distributed within the event horizon. If major disagreements with the no-hair theorem were happening, this system would show it.
"It is important to black hole scientists that we prove or disprove the no-hair theorem. Without it, we cannot trust that black holes as envisaged by Hawking and others exist at all," said Mauri Valtonen, an astrophysicist at the University of Turku in Finland and a co-author on the paper.
The complex dance of these two incredible objects is not just a cosmic spectacle, but a unique window in the physics of black holes.