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The Rotational Energy of Black Holes Spits Out Powerful Particle Streams

Researchers are getting closer to answering some heavy questions about how supermassive black holes work

An optical light image of a black hole’s jet from the galaxy M87 (Nasa/European Space Agency/Hubble Heritage Team)
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In the center of galaxies, hulking behemoths hide. These monstrous black holes are infamous for devouring entire stars and ginormous gas cloudsThere’s a lot we don’t know about supermassive black holes—what they look like, what may lurk beyond that point where even light cannot escape the tug of gravity, what to think of the streams of particles that some shoot out at close to the speed of light.

These jets are some of the most powerful phenomena in the universe. They can be a thousand times brighter than the entire Milky Way and shine all along the electromagnetic spectrum, from radio waves to gamma rays. (By the way, don’t worry about being blinded or annihilated by jets from our own galaxy: the black hole there is relatively quiet.) 

We still aren’t sure what gives the jets this stunning power, but a team of researchers thinks they are narrowing in on a reason. The jets get an energy boost from the rotational energy of the black hole, they say in a paper published in Nature

Supermassive black holes pull in gas, dust and other interstellar material because of their gravity. This action creates a disk of material—called an accretion disk—that surrounds the black hole. Matter from those disks might power the jets, researchers think.

To investigate further, the research team looked at data for 217 supermassive black holes. They noticed that black holes with brighter accretion disks have more powerful jets—clear evidence of a correlation between the two. However, "most of the jets were producing 10 times that of their accretion disks," reports Daniel Clery for Science.

That discrepancy lends support to the idea that the black hole’s spin plays a role, according to the lead astrophysicist, Gabriele Ghisellini of the National Institute for Astrophysics in Merate, Italy. Cleary writes:

The most popular explanation of how jets form is that the fast-spinning accretion disk, which contains charged particles, will produce a powerful magnetic field that is in contact with the black hole. If the black hole is spinning, it drags on the field, winding it into a tight cone at the rotational poles of the black hole. It is this twisted field that accelerates particles away from the black hole as jets and, in the process, extracts energy from the rotation of the black hole.

But, the case isn’t closed yet. "The next step for science is to measure the spin of a black hole," Ghisellini told Science. That step isn’t possible yet, but according to an article on Phys.org, "that should change in 2028 when a European project, the Athena x-ray observatory goes aloft." 

The observatory may help answer a lot of questions about black holes and the structure of the cosmos, perhaps including why the black hole jets seem to line up with the large scale of the universe—filaments of the cosmic web. We also need to figure out how this one black hole got "kicked out of its home galaxy." And those are just the questions raised by findings in the past week. Black holes really are one of the universe’s most enormous mysteries.

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