Astronomers have for the first time found evidence of a binary system of supermassive black holes orbiting each other. It was possible to find them thanks to the signals obtained from the jets released as a result of the accumulation of matter in the two black holes.
This fascinating phenomenon was discovered in the galaxy OJ 287, located in the direction of the constellation Cancer, about 5 billion light-years away. Astronomers have been following it since 1888. More than 40 years ago, astronomer Aimo Sillanpa of the University of Turku and his colleagues noticed that there is an interesting pattern of two cycles in the radiation of this galaxy: one lasting about 12 years, the second about 55 years. Scientists have hypothesized that these two cycles are the result of the orbital motion of two black holes orbiting each other. A shorter cycle may be an orbital cycle, while a longer cycle may be the result of slow evolution of orbital orientation.
The orbital motion is manifested by a series of bursts that occur when the secondary black hole sinks into the primary black hole's accretion disk at slightly less than the speed of light. As the secondary black hole sinks in this way, the material in the disc heats up and the hot gas is ejected as expanding bubbles. It takes these hot bubbles months to cool, before they emit light and produce a flash of light that is brighter than the light of a trillion stars and lasts for about two weeks.
For decades, scientists led by Mauri Valtonen of the University of Turku in Finland and his colleague Achamveedu Gopakumar of the Tata Institute for Fundamental Research in Mumbai, India, have been trying to estimate the time it takes for a secondary black hole to fall into an accretion disk. Eventually, they were able to model the orbit and predict exactly when these flares would occur.
Successful studies in 1983, 1994, 1995, 2005, 2007, 2015 and 2019 allowed scientists to see the predicted flares and confirm the existence of a pair of supermassive black holes in the OJ 287 galaxy.
"The total number of predicted outbreaks is 26, and almost all of them have been observed. The larger black hole of the pair is more than 18 billion times the mass of our Sun, while its companion is about 100 times lighter. Their orbit is elongated, not circular," said Professor Ahamvidu Gopakumar.
So far, astronomers have not been able to detect a direct signal from the smaller of these two black holes. Until 2021, its existence could only be guessed indirectly, judging by the flares and how it causes the jet of the larger black hole to oscillate.
Observations of the galaxy OJ 287 in 2021/2022, using a large number of different types of telescopes, allowed researchers to obtain evidence for the first time of the presence of a second black hole passing through the accretion disk and the signals from it.