The active galaxy OJ 287 is located in the direction of the constellation Cancer at a distance of approximately 5 billion light-years and has been observed by astronomers since 1888.
Supermassive black holes weighing several billion times the mass of our sun are present at the centers of active galaxies. Astronomers observe them as glowing galactic nuclei where the galaxy’s supermassive black hole gobbles up matter in a violent vortex called an accretion disk. Part of the matter is squeezed out in a powerful jet. This process causes the galactic nucleus to glow brightly across the entire electromagnetic spectrum. In a recent study, astronomers found evidence of two supermassive black holes revolving around each other through signals coming from the jets associated with the accumulation of matter in both black holes. The galaxy, or quasar as it is technically called, is called OJ287 and is further studied and best understood as a binary system of black holes. In the sky, black holes are so close together that they merge into a single point. The fact that the dot actually consists of two black holes becomes apparent by detecting that it emits two different types of signals. The results have been published in the Monthly Notices of the Royal Astronomical Society
The active galaxy OJ 287 lies in the direction of the constellation Cancer at a distance of about 5 billion light-years and has been observed by astronomers since 1888. More than 40 years ago, astronomer from the University of Turku Aimo Sillanpää and his associates noted that there is a prominent pattern in their issuance that has two cycles, one of about 12 years and the longest of about 55 years. They suggested that the two cycles result from the orbital motion of two black holes around each other. The shortest cycle is the orbital cycle and the longest results from a slow evolution of the orientation of the orbit.
The orbital motion is revealed by a series of flares that arise as the secondary black hole regularly plunges through the primary black hole’s accretion disk at speeds that are a fraction slower than the speed of light. This submergence of the secondary black hole heats the material in the disk, and hot gas is released as expanding bubbles. These hot bubbles take months to cool as they radiate and cause a flash of light, a flare, that lasts about a fortnight and is brighter than a trillion stars. After decades of efforts to estimate the timing of the secondary black hole’s fall through the accretion disk, astronomers at the University of Turku in Finland led by Mauri Valtonen and collaborator Achamveedu Gopakumar of the Tata Institute for Fundamental Research in Mumbai, India, and others were able to model the orbit and accurately predict when these eruptions would occur.
Successful observing campaigns in 1983, 1994, 1995, 2005, 2007, 2015, and 2019 allowed the team to observe the predicted flashes and confirm the presence of a pair of supermassive black holes in OJ 287. “The total number of predicted eruptions now stands at 26, and almost all of them have been observed. The largest black hole in this pair weighs more than 18 billion times the mass of our sun, while the companion is about 100 times lighter and its orbit is oblong, not circular,” says Professor Achamveedu Gopakumar. Despite these efforts, astronomers were unable to observe a direct signal from the smaller black hole. Before 2021, its existence had been inferred only indirectly from the flares and the way it causes the largest black hole’s jet to wobble. “The two black holes are so close to each other in the sky that one cannot see them separately, they merge into a single point in our telescopes. Only if we see clearly separate signals from each black hole can we say that we actually have ‘I’ve seen them both’, says lead author Professor Mauri Valtonen. Smallest black hole directly observed for the first time Excitingly, observing campaigns in 2021/2022 at OJ 287 using a large number of telescopes of various types allowed researchers to obtain observations of the secondary black hole traversing the accretion disk for the first time, and the signals emerging from the black hole. smaller. itself. “The period of 2021/2022 had a special meaning in the study of OJ287. It had previously been predicted that during this period the secondary black hole would plunge through the accretion disk of its more massive companion. This dip was expected to produce a very blue flash just after impact, and was indeed observed, within a few days of the predicted time, by Martin Jelinek and his associates at the Czech Technical University and the Czech Astronomical Institute,” he says. Professor Mauri Valtonen.
However, there were two big surprises: new types of flares that had not been detected before. The first of these was seen only by a detailed observing campaign conducted by Staszek Zola of the Jagiellonian University in Krakow, Poland, and for good reason. Zola and his team observed a huge flare, which produced 100 times more light than an entire galaxy, and lasted only one day. “According to estimates, the flare occurred shortly after the smaller black hole received a massive dose of new gas to swallow during its fall. It is the swallowing process that leads to OJ287’s sudden glow. This process is believed to have powered the jet shooting out of OJ 287’s smaller black hole. An event like this was predicted ten years ago, but has not been confirmed until now,” explains Valtonen. The second unexpected signal came from gamma rays and was observed by NASA’s Fermi telescope. The largest gamma-ray flare at OJ287 for six years occurred just as the smaller black hole dove through the primary black hole’s gas disk. The jet from the smaller black hole interacts with the gas in the disk, and this interaction leads to the production of gamma rays. To confirm this idea, the researchers verified that a similar gamma-ray flare had already occurred in 2013 when the small black hole last passed through the gas disk, seen from the same viewing direction. “So what about the one day burst, why haven’t we seen it before? OJ287 has been recorded in photographs since 1888 and has been closely followed since the 1970s. Turns out we’ve just been unlucky. No one observed OJ287 on exactly those nights when he did his one-night stand. And without the intense follow-up from Zola’s group, we would have missed it this time as well,” says Valtonen.