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An experiment using proton beams to probe how plasma and magnetic fields interact may have solved the mystery of how quasars and active supermassive black holes emit their relativistic jets.
Let’s picture the scene at the heart of a quasar. Super huge black holeperhaps hundreds of millions – or even billions – sometimes the the mass of our sunravenously eating matter that is streaming into its maw from a spiral, ultra-hot disk. that charged material is called plasma, and it is gravitationally drawn into the black hole’s surroundings — however, the black hole does not swallow all of the plasma, which is made of ionized or electrified atoms, which have been stripped of electrons. In fact, the black hole bites off more than it can chew, and some of the plasma is dispersed in jets that collide with the black hole’s powerful magnetic field before that plasma gets anywhere. near the event horizonwhich is basically the point of no return.
These jets can stretch for miles light year into space. However, explaining the physics that occurs at the bottom of the jet, where they are created, has baffled scientists.
The answer may have come from researchers at the Prison Plasma Laboratory (PPPL) in New Jersey, who were able to devise a modification of a plasma measurement technique called proton radiography.
In their experiment, the researchers first created a high-energy-density plasma by firing a pulsed 20-joule beam onto 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 released proton bursts 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 pushes the protons into many discrete 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 the plasma hits them. The X-ray burst acts as a check — because the X-rays pass cleanly through the mesh and the magnetic field, they provide an undistorted image of the plasma to compare with the proton beam measurements.
“Our experiment was unique because we could see the magnetic field changing directly over time,” said Will Fox, the experiment’s principal investigator, in a statement. “We could observe directly how the field is pushed out and responds to the plasma in a kind of tug-of-war.”
They observed in detail the magnetic field bending under pressure from the expanding plasma, and the plasma sloshing against the magnetic field lines. This bubbling and blocking of the plasma is known as magneto-Rayleigh Taylor instability, and has created shapes in the magnetic field that resemble vortices and mushrooms. Crucially, as the plasma energy decreased, the magnetic field lines were able to recede. This compressed the plasma into a narrow straight column, not unlike a relativistic quasar jet.
“When we did the experiment and analyzed the data, we found that we had something big,” said PPPL’s Sophia Malko. “The magneto-Rayleigh Taylor instability has long been thought to arise from the interaction of plasma and magnetic fields but has not been directly observed until now. This observation helps confirm that this instability occurs when an expanding plasma meets magnetic fields .”
The experiment strongly shows that quasar jets can thank this type of reaction of magnetic fields to the expanding plasma for their creation. If the results are a reflection of what happens around active black holes, it would mean that in the accretion disk of the black hole, conditions become so intense that the plasma in the disk is able to push against the packed magnetic field lines strictly, which is then possible. Jump back and push the plasma into a narrow column, almost squirting it out of the black hole. If true, this could be a huge missing piece in our picture of how active black holes work.
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“Now that we have measured these instabilities very precisely, we have the information we need to improve our models and potentially simulate and understand astrophysical jets to a higher degree than before,” a Malko said. “It’s interesting that people can do something in a laboratory that is usually in space.”
The results were published on 27 June in the journal Physical Review Research.
