Galactic ‘Lightsabers’: Answering Longstanding Questions About Jets from Black Holes

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Lee Sandberg

The one thing everyone knows about black holes is that absolutely everything nearby gets sucked into them.

Almost everything, it turns out.

“Even though black holes are defined as objects from which nothing can escape, one of the astonishing predictions of Einstein’s theory of relativity is that black holes can actually lose energy,” says Eliot Quataert, Princeton astrophysicist and current Member in the IAS School of Natural Sciences. “They can rotate, and just like a spinning top slows down over time and loses that energy in its rotation, a rotating black hole can also lose energy to its surroundings.”

Scientists have widely accepted this model since the 1970s. They knew that magnetic fields probably extracted energy from spinning black holes—they just didn’t know how.

A team of astrophysicists from IAS, Princeton University, and Vanderbilt University has now determined conclusively that energy close to the event horizon of M87’s central black hole is pushing outward, not inward. They have also created a way to test the prediction that black holes lose rotational energy, Quataert said, and to establish that this energy produces “the incredibly powerful outflows we see that we call jets.”

twisting magnetic field lines
Andrew Chael, George Wong, Alexandru Lupsasca and Eliot Quataert, Princeton Gravity Initiative
Astrophysicists have discovered that the twisting magnetic field around a black hole determines the tell-tale polarization spiral observed in black hole images. In particular, the direction of energy flow (from the hole to the field or vice versa) determines the how the polarization twists. By measuring which way the polarization spirals, one can infer whether the magnetic field is extracting spin energy from the hole or pumping spin energy into it.

These energy outflow jets “are basically like million-light-year-long Jedi lightsabers,” said his former postdoctoral fellow Alexandru Lupsasca, and they can extend 10 times longer than the Milky Way galaxy.

The results of their work appear in the current issue of The Astrophysical Journal. Andrew Chael, an associate research scholar in astrophysics, is the first author on the paper. He and co-author George Wong, current Frank and Peggy Taplin Member in the School of Natural Sciences, are both members of the Event Horizon Telescope team and have played a critical role in developing the models that are used to interpret black holes.

The team gave Chael credit for the vital insight at the core of the new paper: that the direction in which the magnetic field lines are spiraling reveals the direction of the energy flow. From that, “the rest sort of fell into place,” Quataert said.

“If you took the Earth, turned it all into TNT and blew it up 1,000 times a second for millions and millions of years, that’s the amount of energy that we’re getting out of M87,” said Wong.

Scientists have known for decades that as a black hole starts to spin, it drags the fabric of spacetime around with it. Magnetic field lines that thread through the black hole get dragged along, and that slows down the rotation, leading to the energy release.

“Our new, sharp prediction is that whenever you look at an astrophysical black hole, if it has magnetic field lines attached to it, there will be energy transfer—truly insane amounts of energy transfer,” said Lupsasca, assistant professor of physics and mathematics at Vanderbilt University.

While the energy flow close to the event horizon of M87’s central black hole is streaming outwards, the team said that the energy flow could theoretically go inward in a different black hole. They are confident in their link between energy flow and the direction of the magnetic field lines, and their prediction that the energy flow comes from the black hole will be tested with the launch of the still-theoretical “next generation” Event Horizon Telescope.

For the past year and a half, black hole researchers around the world have been proposing specs for the future instrument, Wong said. "It is incredibly exciting! Linking energy outflow to such a simple observable like this is a critical step on the path to obtaining direct observational evidence of black hole energy extraction. I look forward to seeing what kind of robust statements we'll be able to make with the next generation of black hole images—and movies!"

The four researchers stressed in their paper that they haven’t conclusively shown that the black hole’s spin “truly powers the extragalactic jet,” though the evidence certainly leans in that direction. Even though the levels of energy that their model shows are commensurate with what the jets need, they couldn’t rule out the possibility that the jet could be powered by rotating plasma outside the black hole. “I think it’s extremely likely the black hole powers the jet, but we can’t prove it,” said Lupsasca. “Yet.”

Simple Model and Simulation
Andrew Chael, George Wong, Alexandru Lupsasca and Eliot Quataert, Princeton Gravity Initiative
To test the connection between black hole images and energy flow around black holes, the team used both simple models of a glowing ring of gas (left) and full 3D supercomputer simulations (right). By verifying that the connection between the spiral of polarization in the images persisted in both cases, they established that it could be potentially be used with real images of black holes from the Event Horizon Telescope (EHT) to test if magnetic fields are extracting energy and spinning down the black hole.

This news is adapted from a press release issued by Princeton University.

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