Blazars occupy an intriguing spot in the cosmic zoo. They’re bright active galactic nuclei (AGN) that blast out cosmic rays, are bright in radio emission, and sport huge jets of material traveling in our direction at nearly the speed of light. For some blazars, their jets look curvy and snaky and astronomers have questions.
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“We present evidence and discuss the possibility that it is in fact the precession of the jet source, either caused by a supermassive binary black hole at the footpoint of the jet or – less likely – by a warped accretion disk around a single black hole, that is responsible for the observed variability,” said Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany.
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Britzen and the team investigated an object called OJ 287 to see if it could give some clues. It appears to have two black holes—essentially a black hole binary—at its core. Studies of this galaxy and 12 other AGNS led to the conclusion that jet curvature may provide a smoking gun clue to the existence of binary black holes in galaxy cores.
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One black hole is emitting the jet and the other one’s gravitational influence affects the appearance and behavior of the jet. According to Michal Zajacek, who is a co-author of the study with Britzen, it helps explain the jet’s appearance. “Physics of accretion disks and jets is rather complex but their bulk kinematics can be compared to simple gyroscopes,” he said. “If you exert an external torque on an accretion disk, for instance by an orbiting secondary black hole, it will precess and nutate, and along with it the jet as well, similar to the Earth’s rotation axis that is affected by the Moon and the Sun.”
A magnetized radio jet (yellow), precessing due to a pair of supermassive black holes. The larger one is (black) at the center of the accretion disk. It contains warmer (blue) and cooler (red) gas. The white arrow indicates the spin of the larger black hole. The second black hole orbits (orange) around the central supermassive black hole and the orange arrow shows the orientation of its orbital angular momentum. Due to misalignment, torque from the secondary drives the precession of the accretion disk as well as the launched jet (green circle and arrows). White curved lines indicate radio emission. Courtesy: Michal Zaja?ek/UTFA MUNI
Searching for the Black Hole Binaries
If this is the case for other blazars, the meandering jet and brightness variability may well be the clue astronomers need to probe for other binary black holes. It’s not an easy task to find the black holes, even though the AGNS themselves are bright, according to Britzen. “We still lack the sufficient resolution to probe the existence of supermassive binary black holes directly,” she said. “But jet precession seems to provide the best signature of these objects, whose existence is expected not only by the black hole / AGN community but also from the gravitational wave/pulsar community who recently published evidence for the existence of a cosmic gravitational background due to the gravitational waves emitted by the mergers of massive black holes through cosmic history.”
