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An international team of scientists, including Jacco Vink from the University of Amsterdam, has used NASA's Chandra X-ray Observatory – one of the most sophisticated X-ray observatories built to date – to reveal how very high energy (VHE) gamma-rays are produced by cosmic rays accelerated inside the 30 Doradus C superbubble, the only known VHE gamma-ray superbubble. They showed that the gamma-ray radiation is caused by electrons rather than by very energetic atoms. This poses some problems for the theory that the very high energy cosmic rays come from superbubbles instead from supernova remnants.

False-colour Chandra image of 30 Dor C

The origin of cosmic rays, energetic particles that bombard Earth

The Earth is continuously being bombarded with very energetic particles, consisting mostly of atomic nuclei. These particles are referred to as cosmic rays and would be very damaging for life on earth, were it not for our atmosphere and the earth’s magnetic field. Most of these particles are thought to come from the very large shock waves created by supernova explosions. The only problem with this theory is that, although  supernova remnants are proven to generate cosmic rays, they are not capable of accelerating these particles to the very high energies we sometimes observe on Earth. One alternative explanation for these very high energy particles is that they are generated in so-called superbubble; a very hot bubble of gas created by a succession of supernovae.

Are superbubbles responsible for the high energy cosmic rays?

The theory that superbubbles are the cause of very high energy cosmic rays gained more credence with the detection of very energetic gamma-ray radiation from the spectacular superbubble 30 Doradus C in the nearby galaxy known as the Large Magellanic Cloud. Within this superbubble, one supernova alone seems to be responsible for a shock moving with 3000 km/s. Its gamma-ray photons have energies of more than a Tera electron-Volt (TeV, similar to energies that the Large Hadron Collider at CERN utilizes), proving that in this superbubble, the combined acceleration of particles can reach 10 TeV or more and therefore that it is possible to create these high energy cosmic rays in a superbubble.

EPIC-PN false colour X-ray image of the 30 Doradus region in the Large Magellanic Cloud

To prove that the high energy cosmic rays indeed come from superbubbles, the team had to identify whether the gamma-rays are coming from energetic atoms, which would be proof that the gamma-rays originate in the superbubble, or from electrons. We already know that there are energetic electrons, since the X-ray radiation they produce has been detected. But the question is whether the electrons also cause the gamma-ray radiation. For that to be the case the magnetic field strength in the superbubble needs to be weak. If the magnetic strength is strong the gamma-ray radiation can only be explained by the presence of very high energy atoms. And here is where the new results proved to be important.

Measuring magnetic fields

The team used a method developed by Jacco Vink to measure magnetic fields from X-ray radiation. The higher the magnetic field, the faster very-high energy electrons lose their energy. The electrons keep gaining energy as long as they remain close to the shock front. But as they move away from the shock front they lose energy, becoming invisible in X-rays. This means that for weak magnetic fields the electrons can wander from the shock front a long distance and still shine in X-rays, whereas for strong magnetic fields, the X-ray emission should only occur close to the shock front. However, for a weak magnetic field more electrons are needed to produce X-ray radiation, and then these many electrons also will be responsible for the gamma-ray radiation.

Superbubbles accelerate electrons, but do they accelerate atoms?

What the team found is that the magnetic field is a 100,000 times smaller than the Earth magnetic field, and that these electrons should then also be responsible for the gamma-ray emission. This means that one cannot conclude that cosmic rays are caused by atoms accelerated in the superbubble. Since electrons are accelerated it is still likely that atoms are also accelerated in the superbubble, but hard proof for that is now still lacking. Future observations with the next generation gamma-ray observatory CTA is necessary to see whether there is perhaps an additional contribution of atoms to the gamma-ray radiation.

Publication details

Magnetic field estimates from the X-ray synchrotron emitting rims of the 30 Dor C superbubble and the implications for the nature of 30 Dor C’s TeV emission. Patrick J. Kavanagh, Jacco Vink, Manami Sasaki, You-Hua Chu, Miroslav D. Filipovi, Stefan Ohm, Frank Haberl, Perica Manojlovic, and Pierre Maggi.

Link to the original article (preprint)