16.02.2013

Proof: Cosmic Rays Are Accelerated in Exploding Stars

Particle Decay 'Smoking Gun' Settles Long Debate

A new study confirms what scientists have long suspected: Most cosmic rays – energetic particles that impact on Earth from all directions – are born in the violent aftermath of supernovas, exploding stars throughout the galaxy. A research team led by scientists at the Kavli Institute for Particle Astrophysics and Cosmology at the U.S. 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 cosmic rays are born.

When a star explodes, it sends shock waves racing through space that can accelerate protons to cosmic ray energies. Illustration: Greg Stewart/SLAC

Reporting in the journal "Science", the team, including researchers from Deutsches Elektronen-Synchrotron DESY, identified two ancient supernovas 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.

"It is the first evidence resting only on the fundamental properties of particles that supernova remnants are an important source of protons and heavier nuclei in the cosmic radiation," explained DESY scientist Markus Ackermann, a member of the research team. "The discovery of this specific signature in the gamma radiation from both supernova remnants enables us to diagnose the physics that gives rise to these gamma rays."

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 hazards 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.

“The energies of these protons are far beyond what the most powerful particle colliders on Earth can produce,” said Stefan Funk, an astrophysicist with the Kavli Institute and Stanford University, who led the analysis. “In the last century we’ve learned a lot about cosmic rays as they arrive here. We’ve even had strong suspicions about the source of their acceleration, but we haven’t had unambiguous evidence to back them up until recently.”

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 in surrounding clouds of dust or gas can create particles called pions. A fraction of these 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. "With the detection of this pion cut-off we finally have the long sought direct evidence that cosmic rays are accelerated in supernova remnants," said DESY scientist Rolf Bühler from the research team. "The next step is now to study this acceleration process in more detail to understand to which maximum energy these particles can be accelerated."

"The source of cosmic rays has long remained a mystery after the phenomenon was discovered a hundred years ago," commented astrophysicist Christian Stegmann from the DESY directorate, spokesperson of the astroparticle physics program of the Helmholtz Association. "It seems fitting that supernovae have now finally been nailed down as an important source of these energetic particles, just after we celebrated the centennial of the discovery of cosmic rays not far from here.

Cosmic rays were first discovered in 1912 by the Austrian-American researcher Victor Hess in a series of balloon flights out of Bad Saarow, a Brandenburg town located  about 40 kilometres South-East of today's DESY campus in Zeuthen near Berlin.

Reference: "Detection of the Characteristic Pion-Decay Signature in Supernova Remnants"; M. Ackermann et al.; "Science" (2013); DOI: 10.1126/science.1231160

This is an extended version of the corresponding SLAC press release