Black holes may explain 60-year cosmic rays mystery
Scientists at the Norwegian University of Science and Technology propose that winds from supermassive black holes could accelerate ultra-high-energy cosmic rays, solving a puzzle dating back to 1962. These particles, mostly atomic nuclei, reach energies up to 10^20 electron volts. The hypothesis suggests these winds, moving at half the speed of light, fling particles across the cosmos.
Ultra-high-energy cosmic rays have puzzled physicists since their detection in 1962. Unlike typical cosmic rays, which are atomic nuclei accelerated in extreme environments like supernovae or neutron stars, these rare particles carry far more energy, with sources unknown until now.
A team from the Norwegian University of Science and Technology (NTNU) offers a new explanation in a study published in the Monthly Notices of the Royal Astronomical Society. Lead author Domenik Ehlert, a PhD research fellow at NTNU's Department of Physics, along with associate professor Foteini Oikonomou and postdoctoral fellow Enrico Peretti from Université Paris Cité, focuses on astroparticle physics.
"We suspect that this high-energy radiation is created by winds from supermassive black holes," said Oikonomou.
Active supermassive black holes, unlike the dormant Sagittarius A* at the Milky Way's center, consume matter equivalent to several Suns per year. A fraction of this material forms outflows at up to half the speed of light, known for about a decade to influence galaxy formation by dispersing gas and halting star birth.
"A tiny portion of the material can be pushed away by the force of the black hole before it is pulled in. As a result, around half of these supermassive black holes create winds that move through the universe at up to half the speed of light," Peretti explained.
The researchers propose these winds accelerate protons or atomic nuclei to 10^20 electron volts—roughly the energy of a tennis ball served by Serena Williams at 200 km/h, or a billion times that of particles in the Large Hadron Collider.
"It is possible that these powerful winds accelerate the particles that create the ultra-high-energy radiation," said Ehlert.
While previous ideas pointed to gamma-ray bursts, starburst galaxies, or plasma outflows, none provided conclusive evidence. The NTNU model aligns with the particles' chemical composition in a specific energy range and could be tested via neutrino experiments.
"Our answer is more of a cautious 'maybe'," Oikonomou noted. "We find that the conditions related to these winds align particularly well with particle acceleration. But we are still unable to prove that it is specifically these winds."
These rays pose no threat on Earth due to atmospheric destruction but highlight radiation risks for space travel, though low-energy solar radiation is the primary concern for astronauts.