Supermassive black holes, weighing billions of times the mass of our sun, reside at the centers of active galaxies. These galaxies exhibit bright cores where the supermassive black holes consume matter from an accretion disk, resulting in a violent whirlpool. Some of the matter is expelled in the form of powerful jets, causing the galactic core to emit radiation across the electromagnetic spectrum.
Recently, astronomers made a significant discovery by detecting signals from the jets associated with the accretion of matter into two supermassive black holes. This finding came from studying the galaxy known as OJ287, which is considered a binary black hole system. Despite appearing as a single dot in the sky due to their proximity, the two black holes emit distinct signals, revealing their presence. The research results were published in the Monthly Notices of the Royal Astronomical Society.
OJ287, an active galaxy situated in the constellation Cancer, has been under observation since 1888. Over 40 years ago, astronomers, including Aimo Sillanpää and his colleagues from the University of Turku, noticed a distinctive emission pattern in OJ287 consisting of two cycles: one lasting approximately 12 years and the other approximately 55 years. They proposed that these cycles were the result of the orbital motion of two black holes around each other, with the shorter cycle representing the orbital period and the longer cycle indicating a gradual evolution in the orientation of the orbit.
The orbital motion manifests as a series of flares occurring when the secondary black hole regularly passes through the accretion disk of the primary black hole at speeds slightly lower than the speed of light. As the secondary black hole plunges through the disk, it heats the surrounding material, resulting in the release of hot gas in the form of expanding bubbles. These bubbles take several months to cool down while emitting radiation, leading to a burst of light known as a flare. These flares, lasting around two weeks, are brighter than a trillion stars.
After decades of efforts to estimate the timing of the secondary black hole’s plunges through the accretion disk, a team of astronomers led by Mauri Valtonen from the University of Turku and Achamveedu Gopakumar from the Tata Institute of Fundamental Research in Mumbai successfully modeled the orbit and accurately predicted the occurrence of these flares.
Multiple observational campaigns conducted in 1983, 1994, 1995, 2005, 2007, 2015, and 2019 enabled the team to witness the predicted flares and confirm the presence of a pair of supermassive black holes in OJ287.
“We have now predicted a total of 26 flares, and almost all of them have been observed. The larger black hole in this pair has a mass over 18 billion times that of our sun, while the companion is roughly 100 times lighter. Their orbit is oblong rather than circular,” explains Professor Achamveedu Gopakumar.
Despite these achievements, astronomers had not directly detected signals from the smaller black hole until 2021. Its existence had been inferred indirectly from the flares it caused and the resulting wobbling of the jet from the larger black hole.
“The two black holes are so close in the sky that we cannot observe them as separate entities; they merge into a single point in our telescopes. Only when we can clearly discern distinct signals from each black hole can we claim to have ‘seen’ both of them,” says Professor Mauri Valtonen, the lead author of the study.
Smaller black hole directly observed for the first time
In an exciting turn of events, the observational campaigns carried out in 2021 and 2022 on OJ 287 using a diverse range of telescopes yielded groundbreaking results. These observations provided researchers with the first-ever direct evidence of the secondary black hole plunging through the accretion disk and the subsequent signals emitted by the smaller black hole itself.
“The period in 2021/2022 held significant importance in the study of OJ287. Previous predictions indicated that during this timeframe, the secondary black hole would plunge through the accretion disk of its larger companion. As expected, this event generated a vivid blue flash immediately after the impact, which was observed within days of the predicted time by Martin Jelinek and colleagues from the Czech Technical University and Astronomical Institute of Czechia,” explains Professor Mauri Valtonen.
However, there were two astonishing surprises – novel types of flares that had not been observed before. The first of these flares was captured through an extensive observation campaign led by Staszek Zola from the Jagiellonian University of Cracow, Poland. Zola and his team witnessed a colossal flare, emitting light equivalent to 100 times that of an entire galaxy, and remarkably, it lasted only one day.
“According to estimates, the flare occurred shortly after the smaller black hole received a substantial amount of new gas to consume during its plunge. It is this consumption process that triggers the sudden brightening of OJ287. It is believed that this process energizes the jet emanating from the smaller black hole within OJ287. Although this event was predicted a decade ago, it had not been confirmed until now,” Valtonen explains.
The second unexpected signal originated from gamma rays, which were observed by NASA’s Fermi telescope. The most significant gamma ray flare in OJ287 in six years occurred precisely when the smaller black hole plunged through the gas disk of the primary black hole. The interaction between the jet of the smaller black hole and the gas disk produces these gamma rays. To validate this theory, researchers confirmed that a similar gamma ray flare had occurred in 2013 when the smaller black hole last fell through the gas disk, observed from the same vantage point.
“But why had we not observed the one-day burst before? OJ287 has been documented in photographs since 1888 and has been closely monitored since 1970. It appears that we have simply been unlucky. No one observed OJ287 precisely on the nights when it performed its one-night spectacle. Without the diligent monitoring conducted by Zola’s group, we would have missed it this time as well,” remarks Valtonen.
These endeavors have solidified OJ287 as the leading contender for a supermassive black hole pair emitting gravitational waves at nano-hertz frequencies. Furthermore, OJ287 is regularly monitored by both the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA) consortia, aiming to gather additional evidence for the presence of a supermassive black hole pair at its core and potentially capture a radio image of the secondary jet.
Source: University of Turku