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An experiment that uses proton beams to probe how plasma and magnetic fields interact may have solved the mystery of how quasars and other active supermassive black holes unleash their relativistic jets.
Let’s imagine the scene at the heart of a quasar. A supermassive black holeperhaps hundreds of millions – or even billions – of times The mass of our sunIt greedily swallows matter that flows into its mouth from a spiraling, superheated disk. This charged matter is called plasma, and it is drawn into the black hole by gravity — but not all of the plasma, which is made up of ionized or electrified atoms stripped of electrons, is swallowed by the black hole. In fact, the black hole bites off more than it can chew, and some of the plasma is expelled in jets that are collimated by the black hole’s powerful magnetic field before the plasma gets anywhere. close , event horizonwhich is actually the point of no return.
These jets can extend thousands of meters light year However, explaining the physics that occurs where the jets form has eluded scientists.
The answer may come from researchers at the Princeton Plasma Physics Laboratory (PPPL) in New Jersey, who have modified a plasma-measuring technique called proton radiography.
In their experiments, the researchers first created a high-energy-density plasma by firing a pulsed, 20-joule laser beam at a plastic target. Then, they used powerful lasers to initiate nuclear fusion in a fuel capsule filled with deuterium and helium-3. The fusion reactions emit bursts of protons and X-rays.
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These protons and X-rays then passed through a nickel mesh filled with tiny holes. Think of the mesh as a colander for straining pasta; it filters the protons into many separate beams that can then measure how the expanding plasma plume interacts with a background magnetic field. Because the protons are charged, they will follow the magnetic field lines as they are swept away by the plasma. The X-ray burst acts as a control—since the X-rays pass cleanly through the mesh and magnetic field, they provide an undistorted image of the plasma to compare with the proton beam measurements.
“Our experiment was unique because we could directly see how the magnetic field changed over time,” said Will Fox, the principal investigator of the experiment. expression“We were able to directly observe how the fields were pushed out and responded to the plasma in a kind of tug-of-war.”
They observed in detail the magnetic field, which was bent outward under the pressure of the expanding plasma, as the plasma churned against the magnetic field lines. This bubbling and foaming of the plasma is known as the magneto-Rayleigh Taylor instability, and it created vortices and mushroom-like patterns in the magnetic field. Crucially, as the plasma energy decreased, the magnetic field lines were able to retract. This compressed the plasma into a flat, narrow column not unlike the relativistic jet of a quasar.
“When we did the experiment and analyzed the data, we discovered that we had something big,” said Sophia Malko of PPPL. “Observations of magneto-Rayleigh Taylor instabilities resulting from the interaction of plasma and magnetic fields have long been thought to occur, but until now they had not been directly observed. This observation helps confirm that this instability occurs when expanding plasma encounters magnetic fields.”
The experiment strongly suggests that quasar jets can thank this type of magnetic field response to the expanding plasma for their formation. If the results are a snapshot of what happens around active black holes, it means that in the black hole’s accretion disk, conditions become so intense that the plasma in the disk can push against the tightly packed magnetic field lines, which can then bounce back and push the plasma into a narrow column, almost jetting it away from the black hole. If true, this could be a major missing piece in our picture of how active black holes work.
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“Now that we can measure these instabilities very accurately, we have the information we need to improve our models and simulate and understand astrophysical jets at a much higher rate than before,” Malko said. “It’s interesting that people can produce something in the lab that normally exists in space.”
The findings were published June 27 in the journal Physical Examination Research.