Do dead stars play cosmic pinball?

Supernova remnants could be playing “pinball wizard” with Galactic cosmic rays, accelerating them to high energies. A desert-based project may be close to solving this 100-year old mystery as to the origin of these rays.

Impression of cosmic rays from one source bombarding Earth. Credit: ASPERA

Since their discovery in 1912 by physicist Victor Hess, cosmic rays – 90 per cent of which are atomic particles called protons – have been known to pepper Earth’s upper atmosphere. Some of them are several times more energetic than protons produced in the Large Hadron Collider, the most powerful particle accelerator ever built. And yet for all this time their exact origins have remained speculation.

The biggest problem has been due to the fact that the majority of cosmic rays, being charged particles, have their trajectories mangled up by powerful magnetic fields in the “cosmic accelerators” that they came from. This means that directional information is completely lost.

Supernova remnants, the exploding cores of dead stars, have long been suspected of being one of the sources of high-energy cosmic rays by chucking out relativistic particles like pinballs, into shock fronts and molecular clouds. So the solution would be to look for components of this which are unaffected by magnetic fields, providing a straight-line back to the source. One viable candidate is gamma-ray photons. And therein lies a problem. Gamma rays are absorbed by Earth’s atmosphere – they knock into gas molecules, producing secondary particles. But a quartet of super-sensitive telescopes in the Namibian desert has been patiently using this very effect to its advantage, staring straight into the Galactic plane.

The High Energy Spectroscopic System (HESS – an acronym deliberately chosen to honour Victor Hess) uses highly sensitive detectors to record momentary flashes of visible cherenkov light photons, the optical equivalent of a sonic boom, from these secondary particles in the atmosphere. So HESS detects gamma-rays indirectly, via visible photons. In a new paper, the HESS team have published details showing a correlation of the direction of these gamma-rays (with energies up to a few tens of TeV) with supernova remnants, and any molecular clouds energised by them. This shows that particle acceleration is taking place in these environments.

Molecular clouds are particularly important for this as relativistic protons escaping from a supernova explosion pelt the surrounding cloud, giving it energy and enhancing gamma-ray emissions. When these particular detections were compared with radio emissions, the team found a good match to the clouds’ morphologies.

There still isn’t enough data to give statistically significant results for all the detections. Even though HESS has been operational since 2002, the work is time-consuming as cherenkov flashes are extremely dim and fleeting they can’t be distinguished with the naked eye. But the team hope that the addition of a fifth telescope will enhance the project’s overall sensitivity. Future projects such as the Cherenkov Telescope Array will have resolutions ten times that of HESS, and will inevitably provide better results. But by opening its eyes to cosmic-ray sources, HESS has already shown the way.

The work done is supported by the Namibian authorities, the University of Namibia, the German Ministry for Education and Research (BMBF), the Max Planck Society, the French Ministry for Research, the French National Centre for Scientific Research, the UK Science and Technology Facilities Council (STFC), the Institute for Particle and Nuclear Physics of Charles University (Prague), the Polish Ministry of Science and Higher Education, the South African Department of Science and Technology and National Research Foundation.

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