Scientists Detect Most Powerful Cosmic Rays Ever Found Hitting Earth

The High Energy Stereoscopic System (HESS) in Namibia was instrumental in determining the general origin of cosmic rays. Christian99/Wikimedia, CC BY-SA 3.0

Scientists have detected the most powerful cosmic rays ever found hitting Earth to date, and they’re likely from sources relatively close to Earth.

The rays, which consist of electrons and their antimatter counterparts, positrons, were spotted at energies all the way up to 40 tera-electron volts, or 20 trillion times the energy of visible light.

Researchers detected the rays at the High Energy Stereoscopic System (HESS) observatory in the desert of Namibia. They published their findings on Nov. 25 in the journal Physical Review Letters.

“This is an important result, as we can conclude that the measured [cosmic ray electrons] most likely originate from very few sources in the vicinity of our own solar system, up to a maximum of a few 1000 light years away, a very small distance compared to the size of our Galaxy,” said Kathrin Egberts, the paper’s corresponding author, and head of the experimental astroparticle physics program at the University of Potsdam in Germany.

For reference, the Milky Way—the galaxy that contains our solar system—is about 100,000 light-years across and about 1000 light-years thick.

What Are Cosmic Rays?

Cosmic ray electrons are high-energy particles thought to originate from extreme astrophysical phenomena. These include supermassive black holes, stellar explosions known as supernovae, rapidly spinning ultra-dense neutron stars called pulsars, and potentially other, unknown sources. Scientists say these objects accelerate particles to high energy levels and launch them across the universe.

When the rays collide with Earth’s upper atmosphere, they break into showers of charged particles and emit a signature blue glow called Cherenkov radiation. This phenomenon, similar to how an airplane traveling faster than the speed of sound creates a visible sonic boom, occurs when particles travel faster than the speed of light in any medium other than a vacuum.

The HESS observatory in Namibia uses five large telescopes to scan the upper atmosphere, looking for faint signs of Cherenkov radiation left in the wake of the fast-moving rays. By analyzing over a decade’s worth of data, researchers isolated cosmic ray electrons from other background noise and revealed unprecedented details.

Tracing the Source

Cosmic rays lose energy as they travel through space due to interactions with light and magnetic fields. This means the sources of these highly energetic particles must be relatively nearby for them to retain such power when they reach our planet.

While researchers still can’t pinpoint their exact origin in space, they can narrow down the potential candidates. Likely sources include Monogem Ring, a leftover glow from a supernova that took place around 68,000 years ago; Gamma 2 Velorum, a blue supergiant and one of the brightest stars in the night sky; or a pulsar like Vela or Geminga.

“We were able to put severe constraints on the origin of these cosmic electrons with our detailed analysis for the first time”, said Werner Hofmann, the study’s co-author and a professor at Germany’s Max Planck Institute for Nuclear Physics.