First detection of secondary supermassive black hole in a well-known binary system

At the cores of active galaxies, supermassive black holes can be found that are several billion times more massive than our sun. They are seen by astronomers as the luminous centers of galaxies where the supermassive black hole of the galaxy eats material from a turbulent vortex known as the accretion disk. A portion of the material is forced into a strong jet. The galactic center now radiates intensely over the whole electromagnetic spectrum as a result of this activity.

In a recent research, astronomers discovered signs from the jets connected to the accretion of matter into both black holes that suggested the existence of two supermassive black holes orbiting each other. The galaxy, or quasar as it is officially known, is designated OJ287, and a binary black hole system has received the greatest attention and is the subject of the finest research. The black holes in the sky are so close to one another that they combine as a single dot. By seeing that the dot produces two distinct sorts of signals, it becomes clear that it actually comprises of two black holes. The results have been published in the Royal Astronomical Society's Monthly Notices.

Astronomers have been observing the active galaxy OJ 287 since 1888. It is located in the direction of the constellation Cancer at a distance of around 5 billion light years. Aimo Sillanpää of the University of Turku and his colleagues first observed a distinct pattern in the emission of this star more than 40 years ago. This pattern has two cycles, the shorter of which lasts for around 12 years and the longer for roughly 55 years. They proposed that the two cycles are the consequence of two black holes orbiting one another. The longer cycle occurs from a gradual development of the orbit's orientation, whereas the shorter cycle is the orbital cycle.

A sequence of flares that appear as the secondary black hole repeatedly plunges through the original black hole's accretion disk at speeds that are only a little bit slower than the speed of light indicate the orbital motion. The material in the disk is heated by the secondary black hole's descent, and the heated gas is ejected as expanding bubbles. It takes months for these hot bubbles to cool while they emit a flare that lasts nearly a fortnight and is brighter than a trillion stars.

Astronomers from the University of Turku in Finland, led by Mauri Valtonen and his collaborator Achamveedu Gopakumar from the Tata Institute of Fundamental Research in Mumbai, India, and others, were able to model the orbit and predict the exact timing of these flares after decades of work estimating the secondary black hole's plunge through the accretion disk.

The team was able to witness the expected flares and prove the existence of a supermassive black hole pair in OJ 287 thanks to successful observational missions in 1983, 1994, 1995, 2005, 2007, 2015, and 2019.

"As of right now, 26 anticipated flares have been detected, which is almost the entire amount. According to Professor Achamveedu Gopakumar, the companion black hole in this pair is around 100 times lighter and has an orbit that is oblong rather than circular. The larger black hole in this duo weighs more than 18 billion times the mass of the sun.

Astronomers had not been successful in directly seeing a signal from the tiny black hole despite their best attempts. Prior until 2021, its existence could only be inferred indirectly from the flares and the way it causes the larger black hole's jet to bob.

The two black holes blend into a single point in our telescopes because they are so near to one another in the sky. According to the primary author, Professor Mauri Valtonen, "we can only claim to have genuinely "seen" both black holes if we observe distinctly distinct signals from each one.

First direct observation of a smaller black hole

Excitingly, the observational campaigns on OJ 287 in 2021–2022, which made use of numerous telescopes of various types, enabled scientists to observe the secondary black hole for the first time as it descended through the accretion disk as well as the signals emanating from the smaller black hole.

"The years 2021 and 2022 were particularly important for the research of OJ287. It was previously expected that the secondary black hole will eat through its more massive companion's accretion disk during this time. According to Professor Mauri Valtonen, "This plummeting was projected to result in a strong blue flash immediately following the impact, and it was in fact spotted, days before the estimated time, by Martin Jelinek and associates at the Czech Technical University and Astronomical Institute of Czechia.

There were two major shocks, though: brand-new flare varieties that had never been seen before. Only Staszek Zola from the Jagiellonian University in Cracow, Poland, conducted a thorough observation campaign to witness the first of them, and with good cause. In just one day, Zola and his crew saw a powerful flare that produced 100 times as much light as an entire galaxy.

"According to the estimations, the flare happened soon after the smaller black hole had taken in a significant amount of fresh gas during its dive. The abrupt brightness of OJ287 is caused by the swallowing motion. It is assumed that this process has strengthened the jet that emerges from the OJ 287's tiny black hole. A similar occurrence was anticipated 10 years ago, but it wasn't proven until today, according to Valtonen.

Gamma rays produced the second unexpected signal, which NASA's Fermi telescope saw. Just as the smaller black hole was about to pass through the gas disk of the larger black hole, OJ287 had its largest gamma ray flare in six years. Gamma rays are produced as a result of the interaction between the disk gas and the smaller black hole's jet. To support this theory, the researchers looked for evidence that a comparable gamma ray flare had previously occurred in 2013, when the little black hole had previously passed through the gas disk and been seen from the same vantage point.

"So why haven't we seen the one-day explosion before? What about it? Since 1888, OJ287 has been captured on camera, and since 1970, it has been closely monitored. It turns out that we've just had lousy luck overall. On those nights when OJ287 performed its one-night stunt, nobody specifically saw it. We would have missed it again if Zola's squad hadn't been keeping a close eye on things, according to Valtonen.

OJ 287 is now the top contender for a pair of supermassive black holes that is emitting gravitational waves at nano-hertz frequencies as a result of these studies. Additionally, OJ 287 is frequently observed by the Global mm-VLBI Array (GMVA) and Event Horizon Telescope (EHT) consortia in an effort to find more proof of the existence of a supermassive black hole pair at the object's center and, in particular, to obtain a radio picture of the secondary jet.

Provided by University of Turku