Blazars occupy an intriguing place in the cosmic zoo.
They are bright active galactic nuclei (AGNs) that emit cosmic rays, exhibit bright radio emission, and eject giant jets of matter flying our way at nearly the speed of light.
At some blazars, their jets look curvy and serpentine, and astronomers have questions.
Does the appetite of a blazar’s central black hole affect jets? Is there some internal activity in the accretion disk that is causing the fluctuations in brightness?
Or is something else at play here? It might be more interesting than a black hole (or its accretion disk) doing its job, according to Silke Britzen of the Max Planck Institute for Radio Astronomy in Germany.
“We present evidence and discuss the possibility that it is in fact jet source precession, caused either by a supermassive binary black hole at the jet’s nadir or, less likely, by a warped accretion disk around a single black hole.” is responsible for the observed variability,” said Britzen from the Max Planck Institute for Radio Astronomy in Bonn.
The galaxy with two black holes
The idea of a precessing distorted accretion disk around a supermassive black hole is interesting because this motion could affect the jets. It also plays a role in these periodic changes in brightness.
This effect also occurs in some other galaxies. But what else could be causing the precession? Britzen and the team studied an object called OJ 287 to see if there might be any clues.
It appears to have two black holes at its core – essentially a black hole binary. Studies of this galaxy and twelve other AGNS led to the conclusion that the beam curvature could be a clear indication of the existence of binary black holes in galactic cores.
How would that work? It’s complex, but basically there are two black holes doing an orbital dance at the center of the galaxy. One black hole ejects the jet, and the gravitational influence of the other affects the jet’s appearance and behavior.
It helps explain the jet’s appearance, according to Michal Zajacek, who co-authors the study with Britzen. “The physics of accretion disks and jets are quite complex, but their mass kinematics can be compared to simple gyroscopes,” he said.
“When you apply an external torque to an accretion disk, for example from an orbiting secondary black hole, it precesses and nutates, and with it the jet, similar to the Earth’s axis of rotation, which is influenced by the moon and the sun.”
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 start looking for other binary black holes.
According to Britzen, even if the AGNS themselves are bright, finding the black holes is not an easy task. “We still lack sufficient resolution to directly probe the existence of supermassive binary black holes,” 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 by the gravitational-wave/pulsar community, which recently found evidence for the existence of a cosmic gravitational background.” published the gravitational waves emanating from massive black hole mergers throughout cosmic history.”
Britzen and colleagues used radio observations to study OJ 287 and other prominent AGNs, since these objects are bright radio sources. High-resolution studies used Very Long Baseline Radio Interferometry (VLBI) techniques.
Several telescopes were connected to each other over long distances in order to focus on the very distant galactic nuclei.
This is the same technique that allowed the Event Horizon Telescope (EHT) collaboration to image the shadow of a black hole for the first time and observe the 6.5 billion solar mass black hole in the galaxy M87.
The team also modeled jet precession and a factor called nutation. This is a periodic variation of the jet’s tilt axis.
Think of it as a slight rocking or swaying as the beam progresses. This nutation also hinted at the possible existence of a second black hole. The team applied the model to the OJ 287 jet.
The result seems to strongly point to the binary black hole action underlying this and other AGNS. The compelling evidence is the wavy rays and variability in brightness.
“The blazar variability in many galaxies is likely to be deterministic rather than stochastic in nature,” said Britzen. “Unraveling the inner workings of this black hole machinery using variability studies is fascinating.”
Britzen and the team hope this work will lead to further studies of blazars. In their work, they suggest that others will continue with long-term VLBI observations and studies of the spectral energy distribution of the light from these objects and their jets.
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Written by Carolyn Collins Petersen/ universe today.