New images link cosmic rays to an enormous bubble of gas blown by hot, young stars, implying that these mysterious high-energy particles may be made in the same factories where stars are born.
Cosmic rays have puzzled astronomers since they were discovered nearly a century ago. Most are protons and other atomic nuclei. They bombard our planet from all directions, travelling at close to the speed of light.
Where cosmic rays come from is unknown. Astronomers suspect that supernova explosions boost them to such high speeds. Supernovae happen most often in dense clouds of gas and dust where stars between 10 and 50 times the mass of the sun are born and die – so those could be a good place to look. “We thought there would be a good chance to see cosmic-ray sources in those star-forming regions,” says Isabelle Grenier of the Paris Diderot University in France.
In 2006, astronomers detected a slight excess of cosmic rays coming from the direction of Cygnus X, a star-forming nebula in the constellation Cygnus (the swan).
But that does not necessarily mean that Cygnus X is the source, because magnetic fields force cosmic rays to take a meandering path through the galaxy.
So instead of hunting for cosmic rays directly, Grenier and colleagues looked at gamma rays. Gamma rays are high-energy photons produced when cosmic rays interact with the matter or light around them, and they are not deflected by magnetic fields.
The team turned the Fermi Gamma-ray Space Telescope towards Cygnus X. High-speed winds from massive stars in the nebula have sculpted their surroundings into a set of beautiful interlocking cavities called a superbubble, more than 100 light years across.
The images show this superbubble glowing in gamma rays, forming a bright region that looks roughly like a duck. “An ugly duckling inside the swan,” says Grenier. The spectrum of these gamma rays implies that the cosmic rays are energetic and freshly accelerated, so they must be close to their sources.
In theory, protons and other nuclei can be accelerated to these high energies by shockwaves. Inside Cygnus X, those shockwaves could be generated both by supernovae and by the colliding winds from massive stars.
“It’s the first credible claim of the detection of high-energy gamma ray emission from superbubbles,” says Bob Binns at Washington University in St Louis, who was not involved in the new work. “My belief is when they look carefully at other superbubbles, they will also find this.”
Journal reference: Science, DOI: 10.1126/science.1210311