Fermi spies a superbubble

One of the mysteries I discuss in New Eyes on the Universe is the origin of ultra-high-energy cosmic rays. These are subatomic monsters, particles that smash into Earth’s atmosphere with macroscopic energy: the famous ‘Oh-my-God’ particle carried 3 x 1020 eV – the kinetic energy of a well-struck tennis ball. What mechanism can accelerate a subatomic particle to that sort of energy? No one knows. However, astronomers are closer to understanding the source of cosmic rays with slightly lower energies (up to about 1015 eV – so still far more energetic than anything the Large Hadron Collider can deliver!)

The Fermi space telescope (previously known as GLAST; as I mention elsewhere, thank heavens that not all astronomy missions are known by acronym) has found evidence for the source of at least some medium-to-high-energy cosmic rays. And thought the details are still to be determined it seems that these cosmic rays are accelerated by shock waves produced when supernovae eject material into space. This model of cosmic ray acceleration, appropriately enough, originated with Enrico Fermi.

Artist's impression of the Fermi space telescope

Artist's impression of the Fermi gamma-ray space telescope.
Credit: NASA

What the Fermi telescope actually found was a source of gamma-rays in the constellation of Cygnus. The source lay along a line between two clusters of stars, the clusters being separated by about 160 light years. One cluster contained over 500 massive stars (the sort of stars that form supernovae), the other cluster contained about 75 massive stars. So how does this relate to cosmic rays?

Well, the clusters contain dense gas clouds – that’s an environment in which massive stars are likely to form – but the stellar wind from a massive star pushes the gas away and creates a ‘bubble’ (When a star explodes as a supernova it also creates a ‘bubble’ around what’s left behind.) These bubbles grow and merge with bubbles around other stars and remnants to form ‘superbubbles’. What Fermi detected (the results are published in Science 334 1103-1107) was high-energy gamma-rays coming from a superbubble in Cygnus. (Since gamma-rays aren’t deflected by magnetic fields, they point straight back to their source; Fermi could thus determine the source of these gamma-rays. Cosmic-rays, being electrically charged, are deflected by the magnetic fields in our Galaxy and around Earth; the arrival direction of a cosmic ray does not necessarily point back to its source.)

The best interpretation of the Fermi data is that cosmic rays were being accelerated by shockwaves in the superbubble; whenever those cosmic rays collided with atoms or molecules inside the superbubble, gamma-rays were produced. The gamma-ray energy distribution was what one would expect from such collisions. Furthermore, the spatial distribution followed the shapes of the gas clouds and cavities. So this is good, strong evidence that some cosmic-rays originate from inside massive-star-forming regions of space.

But what precisely is the acceleration mechanism? An isolated shock wave from a single supernova remnant, or the combined effect of many different shocks? It’s not yet clear. As for the source of the ‘Oh-my-God’ particles – well, God alone knows at present.