Cosmic rays, energetic particles that pelt Earth, are born in the violent aftermath of exploding stars – and now we can prove it.
A research team led by scientists at the Kavli Institute for Particle Astrophysics and Cosmology at the SLAC National Accelerator Laboratory sifted through four years of data from NASA’s Fermi Gamma-ray Space Telescope to find the first unambiguous evidence of how these cosmic rays are born.
The team identified two ancient supernovae whose shock waves accelerated protons to nearly the speed of light, turning them into what we call cosmic rays. When these energetic protons collided with static protons in gas or dust they gave rise to gamma rays with distinctive signatures, giving scientists the smoking-gun evidence they needed to finally verify the cosmic-ray nurseries.
Protons make up 90 percent of the cosmic rays that hit Earth’s atmosphere, triggering showers of particles that reach the ground and creating radiation for air travelers. Scientists have theorized that two of the most likely sources for the protons are supernova explosions within our Milky Way galaxy and powerful jets of energy from black holes outside the galaxy. But in neither case had the necessary evidence been nailed down.
That’s because the positively charged protons are deflected by any magnetic field they encounter along the way, so tracing them back to their source is impossible. But researchers using Fermi’s main instrument, the Large Area Telescope, were able to approach the problem straight on through gamma-ray observations.
The supernova shock waves accelerate protons to cosmic-ray energies through a process known as Fermi acceleration, in which the protons are trapped in the fast-moving shock region by magnetic fields.
Collisions between the speeding protons and slower-moving protons, most often in surrounding clouds of dust or gas, can create particles called neutral pions. The pions, in turn, decay quickly into gamma-ray photons, the most energetic form of light. Unaffected by magnetic fields, the gamma rays travel in a straight line and can be traced back to their source. The gamma rays from this particular process come in a distinctive range of energies.
Fermi researchers analyzed data from two supernova remnants thousands of light years away. Both turned out to be strong sources of gamma rays, but not at energies below what neutral pion decay would produce — the observational proof scientists had been looking for.
As humans spend more time high up in and above the atmosphere, many questions remain on the way cosmic rays affect life here on Earth, and the fundamental processes that control their origins and acceleration.
The next step in this research, Funk added, is to understand the exact details of the acceleration mechanism and also the maximum energies to which supernova remnants can accelerate protons.