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Volume rendering of (a) ∣∇ × B∣ and (b) ∣∇ × ΓV∣ (normalized to their respective rms values) taken at t = 2.5lc/c from the reference turbulence simulation (σ = 16, lc/di = 133).
(c) One-dimensional spectra of the turbulent magnetic field (red) and fluid four-velocity (blue) at t = 2.5lc/c. Each spectrum is normalized so that ∑k⊥P(k⊥)=1. A power-law slope of k⊥−5/3 (dashed black line) is shown for reference.
Credit: The Astrophysical Journal Letters (2024). DOI: 10.3847/2041-8213/ad955f
Ultra-high energy cosmic rays, which emerge in extreme astrophysical environments—like the roiling environments near black holes and neutron stars—have far more energy than the energetic particles that emerge from our sun. In fact, the particles that make up these streams of energy have around 10 million times the energy of particles accelerated in the most extreme particle environment on earth, the human-made Large Hadron Collider.
Where does all that energy come from? For many years, scientists believed it came from shocks that occur in extreme astrophysical environments—when, for example, a star explodes before forming a black hole, causing a huge explosion that kicks up particles.
That theory was plausible, but, according to new research published in The Astrophysical Journal Letters, the observations are better explained by a different mechanism. The source of the cosmic rays' energy, the researchers found, is more likely magnetic turbulence. The paper's authors found that magnetic fields in these environments tangle and turn, rapidly accelerating particles and sharply increasing their energy up to an abrupt cutoff.
"These findings help solve enduring questions that are of great interest to both astrophysicists and particle physicists about how these cosmic rays get their energy," said Luca Comisso, associate research scientist in the Columbia Astrophysics Lab, and one of the paper's authors.
The paper complements research published by Comisso and collaborators on the sun's energetic particles, which they also found emerge from magnetic fields in the sun's corona. In that paper, Comisso and his colleagues discovered ways to better predict where those energetic particles would emerge.
Ultra-high energy cosmic rays are orders of magnitude more powerful than the sun's energetic particles: They can reach up to 1020 electron volts, whereas particles from the sun can reach up to 1010 electron volts, a 10-order-of-magnitude difference. (To give an idea of this vast difference in scale, consider the difference in weight between a grain of rice with a mass of about 0.05 grams and a 500-ton Airbus A380, the world's largest passenger aircraft.)
"It's interesting that these two extremely different environments share something in common: their magnetic fields are highly tangled and this tangled nature is crucial for energizing particles," Comisso said.
"Remarkably, the data on ultra-high energy cosmic rays clearly prefers the predictions of magnetic turbulence over those of shock acceleration. This is a real breakthrough for the field," said Glennys R. Farrar, an author on the paper and professor of physics at New York University.
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