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The newly discovered high-energy glow of the Milky Way hints at the secrets of cosmic rays

The Milky Way shines with a gamma-ray fog, with energies that far exceed anything that physicists can produce on Earth, according to a new newspaper. The gamma rays detected in the study, which will be published in Physical Review Letters, came from the entire disk of the galaxy and reached nearly one trillion (1015) volts of electrons, known as petaelectron volt or PeV.

These diffuse gamma rays hint at the existence of powerful cosmic particle accelerators within the Milky Way. Physicists believe that such accelerators are the source of mysterious and highly energetic cosmic rays, charged particles that traverse the galaxy, sometimes landing on Earth. When cosmic rays, which consist mainly of protons, fall against interstellar debris, they can produce gamma rays, a form of high-energy light.

Scientists suspect that certain galactic environments could elevate cosmic ray particles to more than one PeV. In comparison, the Large Hadron Collider, the leading man-made particle accelerator, accelerates protons to 6.5 trillion volts of electrons. But physicists have not definitively identified any natural cosmic accelerator capable of reaching a PeV, known as PeVatrons. One possibility is that the remnants of supernovae, the remnants of exploded stars, harbor shock waves that can accelerate cosmic rays to such energies (SN: 11/12/20).

If PeVatrons exist, the emitting cosmic rays would permeate the galaxy, producing a diffuse glow of extreme energy gamma rays. That’s what the researchers in Tibet’s AS-gamma experiment discovered. “It’s good to see things fit,” says physicist David Hanna of McGill University in Montreal, who did not participate in the study.

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Scientists believe that after cosmic rays leave their birthplaces, they traverse the galaxy, twisted by its magnetic fields. “We live in a bubble of cosmic rays,” says astrophysicist Paolo Lipari of Rome’s National Institute of Nuclear Physics, who was not involved in the research. Because they are not deflected by magnetic fields, gamma rays point to their sources, revealing the whereabouts of the traveling cosmic rays. The new study "provides information on how these particles fill the galaxy."

The lower energy gamma rays also permeate the galaxy. But higher-energy gamma rays are needed to understand the higher-energy cosmic rays. “In general, the higher the energy of gamma rays, the higher the energy of cosmic rays,” says astrophysicist Elena Orlando of Stanford University, who did not participate in the research. "So the detection … tells us that PeV cosmic rays originate and propagate on the galactic disk."

Scientists in the Tibet AS-gamma experiment in China observed gamma rays with energies between about 100 trillion and one trillion volts of electrons from the region of the sky covered by the Milky Way disk. The search for possible sources of the 38 highest-energy gamma rays, above 398 trillion volts, was left empty, supporting the idea that gamma rays came from cosmic rays that had roamed the galaxy. The highest-energy gamma rays carried about 957 trillion volts of electrons.

Tibet's AS-gamma researchers declined to comment on the study.

Scientists have already seen extremely energetic gamma rays from individual sources in the Milky Way, such as the Crab Nebula, a supernova remnant (SN: 06/24/19). These gamma rays are probably produced differently, by electrons that radiate gamma rays as they circulate within the cosmic accelerator.

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