The Milky Way’s newfound high-energy glow hints at the secrets of cosmic rays

The Milky Way glows with a gamma ray haze, with energies vastly exceeding anything physicists can produce on Earth, according to a new paper. Gamma rays detected in the study, to be published in Physical Review Letters, came from throughout the galaxy’s disk, and reached nearly a quadrillion (10¹⁵) electron volts, known as a petaelectron volt or PeV.

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

Certain galactic environments could rev up cosmic ray particles to more than a PeV, scientists suspect. In comparison, the Large Hadron Collider, the premier particle accelerator crafted by humans, accelerates protons to 6.5 trillion electron volts. But physicists haven’t definitively identified any natural cosmic accelerators capable of reaching a PeV, known as PeVatrons. One possibility is that supernova remnants, the remains of exploded stars, host shock waves that can accelerate cosmic rays to such energies (SN: 11/12/20).

If PeVatrons exist, the cosmic rays they emit would permeate the galaxy, producing a diffuse glow of gamma rays of extreme energies. That’s just what researchers with the Tibet AS-gamma experiment have found. “It’s nice to see things fitting together,” says physicist David Hanna of McGill University in Montreal, who was not involved with the study.

After cosmic rays are spewed out from their birthplaces, scientists believe, they roam the galaxy, twisted about by its magnetic fields. “We live in a bubble of cosmic rays,” says astrophysicist Paolo Lipari of the National Institute for Nuclear Physics in Rome, who was not involved with the research. Because they are not deflected by magnetic fields, gamma rays point back to their sources, revealing the whereabouts of the itinerant cosmic rays. The new study “gives you information about how these particles fill the galaxy.”

Lower-energy gamma rays also permeate the galaxy. But it takes higher-energy gamma rays to understand the highest-energy cosmic rays. “In general, the higher the energy of the gamma rays, the higher the energy of the cosmic rays,” says astrophysicist Elena Orlando of Stanford University, who was not involved with the research. “Hence, the detection … tells us that PeV cosmic rays originate and propagate in the galactic disk.”

Scientists with the Tibet AS-gamma experiment in China observed gamma rays with energies between about 100 trillion and a quadrillion electron volts coming from the region of the sky covered by the disk of the Milky Way. A search for possible sources of the 38 highest-energy gamma rays, above 398 trillion electron volts, came up empty, supporting the idea that the gamma rays came from cosmic rays that had wandered about the galaxy. The highest-energy gamma ray carried about 957 trillion electron volts.

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

Scientists have previously seen extremely energetic gamma rays from individual sources within the Milky Way, such as the Crab Nebula, a supernova remnant (SN: 6/24/19). Those gamma rays are probably produced in a different manner, by electrons radiating gamma rays while circulating within the cosmic accelerator.