For the past century, physicists have puzzled over cosmic rays, particles (mostly protons) that hurtle through space at high speed and seem to come from all directions equally. What's the source of these galactic projectiles? And how do they come to be traveling so fast? Today, an international team announced a major step toward answering those questions: conclusive evidence that at least some of the cosmic rays come from supernova remnants—expanding shells of matter from exploded stars—that are acting as natural particle accelerators.
Cosmic rays have proved an enduring mystery because their interactions obscure their origins. Being charged particles, they "feel" the push and pull of magnetic fields in space. As a result, they travel across the galaxy in long, looping paths that make it impossible for detectors on Earth to trace where they've come from.
The speed at which the particles travel suggests that they must come from some violent, high-energy source. Researchers have long suspected supernova remnants but had no way of proving it. "We needed a neutral messenger to see where they originated from," says Stefan Funk of Stanford University in Palo Alto, California, spokesperson for the 170-strong team. Gamma rays—high-energy photons produced as a byproduct of accelerating protons—can fill the role of neutral messengers because they have no electric charge and thus travel through space in straight lines. But high-speed electrons also produce gamma rays, and until now physicists have not been able to tell whether the gamma rays they detect from supernova remnants are coming from electrons or protons. "Disentangling these two has been very difficult," says Luke Drury of the Dublin Institute for Advanced Studies.
The Italian-American physicist Enrico Fermi in 1949 first proposed a way supernova remnants could accelerate protons. The mechanism goes something like this: The supernova remnant is an expanding spherical shell of matter pushing outwards into the diffuse gas between the stars—the interstellar medium. This produces a shock wave at the front of the shell, and this shock front carries along complex magnetic fields, both in front and behind. A charged particle such as a proton in the impacted gas can get bounced back and forth between these two fields, repeatedly passing through the shock front and getting a kick of new energy on each pass. Eventually it will gain enough energy to escape the magnetic fields and shoot off into space as a cosmic ray.
When the high-speed proton collides with one of its low-speed cousins in the interstellar medium, their interaction often spawns an elementary particle called a neutral pion. The pion decays almost immediately into two gamma rays—the neutral messengers that show high-energy protons are present. Electrons accelerated by the supernova remnant also produce gamma rays, but by a different mechanism that leaves a subtle difference in the energy spectra of the two sets of gamma rays. Because the proton's gammas actually come from pions, each gamma ray must have at least half the energy of a pion. Lower energy gamma rays don't appear in their energy spectrum. Gamma rays from electrons, by contrast, don't show that low-energy cutoff point.
Gamma rays from deep space are hard to detect because Earth's atmosphere stops them before they reach the surface. And until recently, orbiting detectors haven't been accurate enough to detect the energy cutoff. But NASA's Fermi Gamma-ray Space Telescope can do it, and Funk's team began using it soon after it was launched in 2008. For the next 4 years they studied two nearby supernova remnants. "The instrument is not perfect, but we could clearly see the cutoff at the right energy," Funk says. "We have unambiguously shown that supernova remnants can accelerate cosmic rays." "This is quite an important and long-expected result," says Werner Hofmann of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. It "settles the case at least for this special class of supernova remnants."
The team has shown that supernova remnants are a source of cosmic rays. But are they the main source? Finding out will require the accumulation of more data and the study of more objects, Funk says, but at least researchers now have the tools they need: "The result is nice in the sense that the theoretical understanding was done a long time ago. Only now do we have the technology to confirm these ideas."