Astronomers detect secondary supermassive black hole in active galaxy OJ 287

An international team of astronomers has directly observed for the first time the second of two supermassive black holes circling each other in the active galaxy OJ 287. This galaxy, located approximately 5 billion light-years away in the constellation Cancer, has been a subject of study since 1888.

Over 40 years ago, astronomers from the University of Turku, led by Aimo Sillanpää, noticed a recurring pattern in OJ 287's emission. They identified two cycles, one lasting around 12 years and the other approximately 55 years. They suggested that these cycles were a result of the orbital motion of two black holes within the galaxy. The shorter cycle corresponded to the orbital period, while the longer one indicated a gradual change in the orbit's orientation.

Despite decades of research and efforts to estimate the timing of the secondary black hole's movement through the accretion disk, astronomers had not directly observed any signals from the smaller black hole. Its existence had only been inferred indirectly from the flares it caused and from the way it makes the jet of the bigger black hole wobble.

However, recent observational campaigns using a multitude of telescopes in 2021 and 2022 provided researchers with groundbreaking evidence of the secondary black hole's plunge through the accretion disk for the first time. These campaigns, led by Mauri Valtonen from the University of Turku and Achamveedu Gopakumar from the Tata Institute of Fundamental Research in Mumbai, India, yielded two major surprises.

The first was an entirely new type of flare detected during a detailed observation campaign conducted by Staszek Zola and his team from the Jagiellonian University of Cracow, Poland. This flare was exceptionally bright, emitting light equivalent to 100 times that of an entire galaxy, and it lasted only one day.

The second unexpected signal was detected in the form of gamma rays by NASA's Fermi telescope. This observation revealed the most significant gamma-ray flare from OJ 287 in six years, occurring precisely when the smaller black hole plunged through the gas disk of the primary black hole. The interaction between the smaller black hole's jet and the disk's gas led to the production of these gamma rays. To validate this idea, researchers confirmed that a similar gamma-ray flare had occurred in 2013 when the smaller black hole last passed through the gas disk, as observed from the same viewing angle.

"OJ287 has been recorded in photographs since 1888 and has been intensively followed since 1970. It turns out that we have simply just had bad luck. Nobody observed OJ287 exactly on those nights when it did its one-night stunt. And without the intense monitoring by Zola’s group, we would have missed it this time as well,” Valtonen said.

The findings will appear in Monthly Notices of the Royal Astronomical.