by Fluctuations in the bright light from the brightest galaxies in the Universe may reveal a secret within them, astronomers say.
According to a new study of a type of galaxy known as a blazar, the best explanation for the strange changes in its brightness are two supermassive black holes trapped in a decaying cycle.
Led by astronomer Silke Britzen of the Max Planck Institute for Radio Astronomy in Germany, an international team studied 12 blazar galaxies, and found that the description of rotating black holes could be applied to all of them.
This may be a clue to how black holes with millions to billions of times the mass of the Sun grow to be so massive. A large number of supermassive black hole binaries may point to the largest merger in the history of the Universe.
“We present the evidence and discuss the possibility that it is the direction of the jet source, either caused by a supermassive black hole in the jet area or – under the accretion disk around the same black hole, that is responsible for the observed differences,” said Britzen.
The brightest known objects in the Universe are quasars – galaxies with a supermassive black hole at their center that ejects material from a large, hot disk around it like water down a stream.
A blazar is a type of quasar that is oriented in a certain direction. As the material orbits the black hole, some is deflected and accelerated by magnetic lines out of the incident area towards the poles, where it is launched into space as the plasma jets approach the speed of light. In the galaxies ofblazar, one of those jets is pointed right at us.
But there is something interesting about the light from blazars: it varies in size. This was previously interpreted as stochastic, or random, possibly the result of blobs of material embedded in the jets, causing what appeared to be flares.
A few years ago, Britzen and his colleagues showed that the variability of the blazar OJ 287 was the result of a previous jet. That means the source of the jet is moving like a rotating surface, causing the jet itself to rotate and separate with it. We also know that OJ 287 hosts two supermassive black holes; precession is linked to their orbit.
OJ 287 is a very difficult case; its variation is very pronounced. But using the same framework for 11 other blazars with less variability, including OJ 287, the researchers found that precession plays an important role in their variability.
Other processes, such as blobs and shocks, may also be involved, but low precession should not be overlooked in explaining why the jets look the way they do, the researchers said.
Astronomer Michal Zajaček of Masaryk University in the Czech Republic says: “The physics of accretion discs and jets is complex but their overall kinematics can be compared to gyroscopes.”
“If you apply an external torque to the accretion disk, for example with a second rotating black hole, it will start and nutate, and with it the jet as well, similar to the axis of rotation of the Earth affected by the Moon and the Sun.”
We currently do not have the tools and resolution to observe the disk structure that will reveal these black holes, but continued monitoring of the core, and long-term observations of other blazars, may continue to bring information about their existence.
“Jet precession seems to provide the best signature of these objects, its presence is expected not only by the black hole / AGN community, but also by the gravitational wave / pulsar community which recently published evidence of the origin of cosmic gravitation due to gravitational waves emitted by merging holes the biggest blacks in the history of the universe,” says Britzen.
The research was published in The Astrophysical Journal.
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