For the first time, astronomers have detected a burst of light from the smaller of the two supermassive black holes (SMBH) in the blazar OJ 287. The smaller SMBH’s orbit regularly causes it to cause bursts in the accretion disk of the larger SMBH, and the pair’s dance is attracting attention astronomers for decades, but this is the first light believed to be from the direct emissions of the smaller black hole
OJ 287 has interested astronomers since 1888, long before we even knew that galaxies were not part of our Milky Way. The smaller black hole orbits the larger one every 12 years, and the angle of its orbit causes it to pass through the accretion disk twice during that period, releasing bursts of light that last about two weeks.
A briefer bright flash outside the galaxy’s regular cycle revealed something unusual. In a new paper, astronomers from Finland and India come together to explain the observations.
Most, if not all, galaxies have SMBHs at their cores. When galaxies merge, black holes usually merge – but before that happens, they spin around each other in an ever-tightening dance. Although we cannot see the black holes themselves, in about a dozen cases we can observe the radiation produced by these interactions as the black holes approach.
OJ287 is one of them, and at five billion years old it is a relatively close example. The SMBHs are also unusually close together. At approximately 18 billion solar masses, the large SMBH is 4,000 times more massive than the one at the heart of our own galaxy and about one-fifth the size of the largest known SMBH. All this makes the system one of the few cases where there are good prospects for detecting gravitational waves from their mutual orbits, but it also prevents us from resolving the disks of the two objects independently.
Forty years ago, Aimo Sillanpää of Finland’s University of Turku made sense of the lightning pattern by identifying two cycles for his Ph.D. One, lasting 12 years, represents the time it takes for the smaller SMBH to orbit the larger one, with two bursts per cycle, as it passes through the primary’s accretion disk, heating it up enough in the process to brighten dramatically.
The longer cycle involves changes in the way the orbit is oriented relative to the disk, such as the way Earth’s orbit slowly evolves into a much longer cycle of stretching and contracting. On this basis, Sillanpää successfully predicted one of the largest eruptions of OJ 287 in 1983. The calculations were complicated because the gravity of the smaller hole distorted the accretion disk of the larger one. However, refinement of the model has resulted in predictions accurate to within a few hours.
However, when a flash was detected by OJ287 in early 2022, it did not fit the pattern identified by Sillanpää, as regular outbursts had occurred in 2015 and 2019. In a paper by Sillanpää and 26 co-authors, out- the successive outburst is attributed to the smaller SMBH capturing new gas as it passed through the disk of the larger hole, eventually engulfing it. “It is the absorption process that causes OJ287 to suddenly brighten,” says lead author Professor Mauri Valtonen of the University of Turku in statement. “This process is thought to have powered the jet coming out of OJ 287’s smaller black hole. An event like this was predicted ten years ago but has not been confirmed until now,”
Valtonen claims that such out-of-cycle bursts have happened before, but they were not detected before because they are so short, lasting only a day. While astronomers have been observing OJ 287 since 1970, after periodic observations of 19th century, we don’t check at all times, so short bursts can be missed. In fact, even the longer flare in 2019 was invisible from Earth because the Sun was in the way, and visible only because of the Spitzer telescope’s lagging orbit.
The study was published in open access in Monthly Notices of the Royal Astronomical Society