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

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Artist’s illustration of OJ287 as a binary black hole system. The secondary black hole of 150 million solar masses moves around the primary black hole of 18 billion solar masses. A gas disk surrounds the latter. The secondary black hole is forced to strike the accretion disk twice during its 12-year orbit. The impact produces a blue flash that was detected in February 2022. In addition, the impact also induces the secondary black hole to have bright bursts of radiation several weeks earlier, and these bursts were also detected as a direct signal from the secondary black hole. Credit: AAS 2018

Supermassive black holes weighing several billion times the mass of our Sun are present at the centers of active galaxies. Astronomers observe them as luminous galactic nuclei in which the galaxy’s supermassive black hole devours matter from 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 shine brightly across the entire electromagnetic spectrum.

In a recent study, astronomers found evidence of two supermassive black holes rotating around each other via signals from jets associated with the accretion of matter into both black holes. The galaxy, or quasar as it’s technically called, is called OJ287 and is studied extensively 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 evident by noting that it emits two different types of signals. The result was published in Monthly Notices of the Royal Astronomical Society.

The active galaxy OJ 287 is located in the direction of the constellation Cancer at a distance of about 5 billion light years and has been observed by astronomers since 1888. Already more than 40 years ago, the University of Turku astronomer Aimo Sillanp ei his collaborators noted that there is a prominent pattern in his emission that it has two cycles, one of about 12 years and the longer one of about 55 years. They suggested that the two cycles result from the orbital motion of two black holes around each other. The shorter cycle is the orbital cycle and the longer one results from a slow evolution of the orientation of the orbit.

The orbital motion is revealed by a series of flares that occur as the secondary black hole regularly dives through the primary black hole’s accretion disk at speeds that are a fraction slower than the speed of light. This plunge of the secondary black hole heats the disk material and the 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 two weeks and is brighter than a trillion stars.

After decades of efforts to estimate the timing of the secondary black hole’s plunge through the accretion disk, astronomers at the University of Turku in Finland led by Mauri Valtonen and his collaborator Achamveedu Gopakumar of the Tata Institute of Fundamental Research in Mumbai, India , and others were able to model the orbit and accurately predict when these flares would occur.

Successful observing campaigns in 1983, 1994, 1995, 2005, 2007, 2015 and 2019 allowed the team to observe the predicted flares and confirm the presence of a pair of supermassive black holes in OJ 287.

“The total number of flares predicted is now 26, and nearly all of them have been observed. The larger 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 their orbit is oblong, not circular,” says Professor Achamveedu Gopakumar.

Despite these efforts, astronomers had been unable to observe a direct signal from the smaller black hole. Prior to 2021, its existence was only indirectly inferred from the flares and the way it swings the larger black hole’s jet.

“The two black holes are so close to each other in the sky that it is not possible to 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 them I’ve seen both,” says lead author Professor Mauri Valtonen.

Smallest black hole observed directly for the first time

Interestingly, observing campaigns in 2021/2022 on OJ 287 using a large number of telescopes of various types allowed the researchers to obtain for the first time observations of the secondary black hole plunging through the accretion disk and signals from from the smallest black hole yes.

“The period in 2021/2022 held special significance in the study of OJ287. Previously, it was predicted that during this period the secondary black hole would plummet through the accretion disk of its more massive companion. This plunge was expected produced a very blue flash immediately after impact, and was actually observed a few days before the expected time by Martin Jelinek and associates from the Czech Technical University and the Astronomical Institute of the Czech Republic,” says Professor Mauri Valtonen.

However, there were two big surprises: new types of flares that hadn’t been detected before. The first of them has only been seen by a detailed observation campaign by Staszek Zola of the Jagiellonian University in Krakow, Poland, and for good reason. Zola and his team observed a large flare, which produced 100 times more light than an entire galaxy, and lasted just one day.

“The flare is estimated to have occurred shortly after the smaller black hole received a massive dose of new gas to swallow during its plunge. It is the swallowing process that leads to the sudden brightening of OJ287. It is thought that this process boosted the jet exiting the smaller black hole of OJ 287. An event like this was predicted ten years ago, but it hasn’t 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 in OJ287 in six years occurred just as the smaller black hole plummeted 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 fell through the gas disk, seen from the same observation direction.

“So, as for the one-day explosion, why haven’t we seen it before? OJ287 has been recorded in photographs since 1888 and has been followed intensely since 1970. It turns out we just had bad luck. No one exactly observed OJ287 on those nights when he pulled his one night stunt. And without the intense monitoring from Zola’s group, we would have missed this time as well,” says Valtonen.

These efforts make OJ 287 the best candidate for a pair of supermassive black holes that are sending out gravitational waves at nano-hertz frequencies. In addition, OJ 287 is regularly monitored by both the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA) consortium to probe for further evidence of the presence of a pair of supermassive black holes at its center and, in particular, to look for to obtain the radio image of the secondary jet.

More information:
Mauri J Valtonen et al, Refining the epoch of arrival of the impact glow OJ 287 2022, Monthly Notices of the Royal Astronomical Society (2023). DOI: 10.1093/mnras/stad922

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Monthly Notices of the Royal Astronomical Society

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