..from Astronomy magazine
In 1912, Austrian-American physicist Victor Hess flew aboard a hot air balloon to 17,600 feet (5,350 meters) and found a fourfold increase in radiation above 9,840 feet (3,000m). This cosmic radiation came from all directions. As the decades passed, researchers learned that these cosmic rays constitute 90 percent of high-energy protons, while electrons and atomic nuclei make up the other 10 percent. But finding the source of these particles, and what gives them their incredible energies, has been difficult.
Our Milky Way Galaxy has a magnetic field, and cosmic-ray particles have electric charge — protons and nuclei are positive while electrons are negative. Charged particles like cosmic rays change their directions when moving through a magnetic field, and thus their original trajectories can’t be traced.
But astronomers have another way to find out where cosmic rays come from. They know that a fast-moving proton (like a cosmic ray) that collides with a proton sitting in interstellar gas will produce an elementary particle called the neutral pion. This particle then decays into two gamma rays, each with a specific energy signature centered at 67.5 million electron volts (MeV; for comparison, visible light has energy between 1.5 and 3 electron volts). A gamma ray has a neutral electric charge, and thus magnetic fields don’t affect it. If you can find gamma rays with that specific energy signature, then you’ve found cosmic rays.
Astronomers have looked for the energy signature in the remnant radiation from an exploded star, or supernova. In such an explosion, a star’s outer shells of material fly away from its core and compress and heat — or “shock” — nearby gas. Scientists have long hypothesized that these shocked-material sites are regions of frequent proton-proton collisions and thus that supernovae are the culprit behind cosmic-ray acceleration. But direct evidence — the specific energy signature — didn’t exist.
In February 2013, scientists reported in the journal Science that they had discovered that energy signature — thus proving supernovae are a source of cosmic rays. NASA’s Fermi Gamma-ray Space Telescope observed the supernova remnants IC 443 and W44 sporadically from August 4, 2008, to July 16, 2012, down to energies of 60 MeV. The team then compared their observations to the energy spectrum expected if neutral pion particles decayed into the gamma rays, and they matched remarkably well.
So how does a supernova remnant energize particles to such high speeds? The shock front has entangled magnetic fields. “Charged particles can get trapped in the vicinity of the shock by these magnetic fields, such that they travel back and forth across the shock itself,” explains Scott Wakely, A University of Chicago particle astrophysicist not involved with the Fermi telescope study. “Each time they pass across, they gain a little energy, and so after a while — thousands of years! — they become quite energetic and can escape, at which point they propagate into the galaxy, becoming cosmic rays.”
A century after Hess’ discovery, scientists have proven what creates these high-energy cosmic rays. But that doesn’t mean they’ve figured out all there is to know about pervading cosmic radiation. The next step is to determine the details of the acceleration technique and also the maximum energy a cosmic-ray proton can attain. Physicists have discovered gamma rays with energies up to a million times that of the gamma rays discussed in the February Science study. How much more energetic can the particles get?