Zum Ursprung der kosmischen Strahlung

Cosmic particle radiation, consisting mainly of high-energy protons and atomic nuclei, is constantly hitting our Earth's atmosphere. As these are charged particles and nuclei, they are deflected by magnetic fields on their way to us in the Milky Way. We are therefore unable to trace their direction of arrival back to their astronomical origin. However, if these particles and nuclei interact with interstellar matter at their point of origin, they are expected to emit high-energy photons, i.e. light in the gamma ray range, which points back to the possible point of origin.

For decades it has been suspected that supernovae, i.e. explosions at the end of the life cycle of massive stars, are the hidden origin of cosmic particle radiation. And indeed, observations with the Fermi space telescope have confirmed that some supernova remnants are massive particle accelerators. But this scenario is not complete: only a few supernova remnants are observed at the highest energies - and these can hardly have accelerated all of the cosmic particles and nuclei that are measured on Earth. Hence, this is the subject of current research into a problem that is more than a hundred years old. One possible explanation is that the best conditions for particle acceleration only prevail in a short period of time after the explosion of a supernova. "Unfortunately, we only expect two to three such explosions per century in our Milky Way," says Olaf Reimer of the Institute for Astro- and Particle Physics. "If we investigate this in galaxies in our cosmic neighborhood, only the most powerful instruments will probably be able to detect the corresponding gamma radiation."

However, May 18, 2023 was a lucky day. A supernova (SN 2023ixf) exploded near us, the Fire Wheel Galaxy (or Messier 101), which is "only" 22 million light years away from Earth. This is the closest supernova from the collapse of a massive star since the launch of NASA's Fermi Space Telescope 15 years ago. Therefore, for the first time, science had the chance to study cosmic particle acceleration just days after the explosion of a supernova. The researchers expected a bright source in the light of the gamma rays, but none was found in the data from the Fermi telescope.

How can such a discrepancy be explained? Supernova remnants should be able to convert up to 10% of their energy into the acceleration of cosmic particles. However, the gamma data, combined with optical observations from the Hubble Space Telescope, suggest that in the case of SN 2023ixf it could not have been more than 1%. But perhaps our models for this are still unrealistic? For example, the predictions change if this explosion was not ideally spherical, but very asymmetrical. Or perhaps we do not yet have a good understanding of the conditions for shock acceleration in the early phase of a supernova. "With the new data from Fermi, we are now challenged to think even harder about whether supernova explosions can still or no longer play the central role in the search for the origin of galactic cosmic particle radiation," says Reimer. The study was led by Guillem Martí-Devesa at the Institute of Astro- and Particle Physics at the University of Innsbruck, who is now a researcher at the University of Trento.

Further information

Early-time gamma-ray constraints on cosmic-ray acceleration in the core-collapse SN 2023ixf with the Fermi Large Area Telescope. G. Martí-Devesa, C. Cheung, N. Di Lalla, M. Renaud, G. Principe, N. Omodei, F. Acero. A&A, DOI: 10.1051/0004-6361/202349061