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Even when it doesn’t detect gamma rays, NASA’s Fermi Gamma-ray Space Telescope helps astronomers learn more about the universe.

The ori­gin of Cos­mic Rays

New results from Gamma-ray astronomy contrast the decade-old standard paradigm for the origin of Galactic cosmic rays. Investigations based on observations with NASA’s Fermi Gamma-ray Space Telescope were led by an Innsbruck researcher.

Cosmic rays are particles (mostly protons and nuclei) accelerated to speeds almost comparable to that of light, travelling through our Galaxy and constantly bombarding Earth’s atmosphere. Being charged particles, their paths are diverted by magnetic fields in our Galaxy. Therefore, we cannot trace their measured arrival direction back to astrophysical objects, which are, the energetic sources that accelerated them in the first place. But when these cosmic rays collide with gas inside or near their sources, they will produce a glow of gamma rays, the most energetic photons in the electromagnetic spectrum.

For decades we have thought that supernovae, the explosions terminating the life of the most massive stars, are those hidden astrophysical sites accelerating cosmic rays. Indeed, Fermi previously found how the remnants of some supernovae can accelerate particles up to extreme energies. But the puzzle is not complete: only very few supernova remnants can do that, and they cannot accelerate all the cosmic rays that we detect on Earth. So, as of today, the origin of Galactic cosmic remains a hot topic. A possible explanation proposed by theorists is that ideal physical conditions occur just for a very short time after a supernova explosion. "Unfortunately, only 2 or 3 supernovae explode every century in the Milky Way," says Olaf Reimer from the Department of Astro- and Particle Physics at the University of Innsbruck, "and we will only be able to see a glow of gamma rays with the most sensitive gamma-ray detectors for other galaxies in our galactic neighbourhood.

In May 18th 2023, however, we were lucky.  A supernova explosion (SN 2023ixf) occurred almost next door, in the Pinwheel Galaxy, a spiral galaxy “only” 22 million light years away from ours. This is the closest supernova arising from the death of a massive star since the start of the Fermi Gamma-ray Space Telescope 15 years ago. Therefore, we have now the first opportunity to experimentally test how cosmic rays are accelerated within a week after such explosion. And for all we know about the early stages of a supernova, at that distance we should have detected a bright energetic glow in gamma-rays - but found no signal at all in the data of Fermi’s Large Area Telescope.

How can that be? Supernova should convert about 10% of their energy into cosmic rays, yet combining our observations with results from optical telescopes like the Hubble Space Telescope we surprisingly find that it cannot be larger than 1%. We then explored several physical ingredients missing in our modelling that can explain that difference and found only two possible reasons: (1) either the explosion and the distribution of gas around it is highly asymmetric and non-spherical, or (2) we do not understand the shock conditions at the early phases of a supernovae. As a result, the investigations led by Guillem Martí-Devesa provides a new, fortunate path towards resolving whether these explosions are or not the missing piece in the puzzle of the origin of Galactic cosmic rays.

Publication: 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

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