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What is cosmic radiation and where does it come from?

Learn about the mysterious origins of cosmic rays and their composition, from charged particles to neutral particles. Discover the possible mechanisms of acceleration and the challenges in studying their high-energy sources.

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What is cosmic radiation and where does it come from?

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  1. What is cosmic radiation and where does it come from? Frederik Rühr, Kirchhoff-Institut für Physik, Universität Heidelberg “The origin of cosmic rays is one of the major unsolved astrophysical problems” (Claus Grupen, Astroparticle Physics, 2005) IRTG Seminar, 26. Oktober 2007

  2. What are cosmic rays?

  3. Why cosmic rays?

  4. What are cosmic rays? • Charged particles (above a given magnetic rigidity) • Protons: 86% • α-particles: 11% • Electrons: 2% • Nuclei, up to uranium: 1% • (Positrons and antiprotons?) • Neutral particles • γ-rays • Neutrinos and antineutrinos (numbers from Perkins, Particle Astrophysics, 2003)

  5. What are cosmic rays? I will focus on this • Charged particles (above a given magnetic rigidity) • Protons: 86% • α-particles: 11% • Electrons: 2% • Nuclei, up to uranium: 1% • (Positrons and antiprotons?) • Neutral particles • γ-rays • Neutrinos and antineutrinos (numbers from Perkins, Particle Astrophysics, 2003)

  6. Where do they come from?

  7. Where do they come from? • Bottom up approach • Charged particles of low energy are accelerated to the observed high energies • Top-down approach • Decay of cosmic strings, topological defects, …

  8. Acceleration of cosmic rays • Time dependant magnetic fields create electric fields that can accelerate charged particles • Cyclotron mechanism • E.g. particles going round in circles around sunspots could be accelerated up to 100 GeV, gaining ~1 GeV per full orbit • Orbits are only stable in the presence of guiding forces • Acceleration by sunspot pairs • Moving pairs of sunspots -> moving magnetic dipole -> electric field • More plausible, but resulting in lower maximum energies

  9. Charged particles traversing shock fronts (e.g. supernova shells) can gain energy • If they transverse the shock front multiple times, due to e.g. being mirrored by magnetic clouds, energies up to 1014Z eV could be reached • Multiple reflection between two shock fronts is also an option • The estimated energy spectrum resulting from this mechanism is expected to run with E-2.1, compared to the -2.7 measured (but the difference can possibly be accounted for by an energy dependence of the escape chance of particles)

  10. Supernova shock fronts (blue)

  11. Energies above 1014 eV? • No mechanisms are known • Can we look for the origin of highly energetic charged primary cosmic rays?

  12. The short answer is: NO! Charged cosmic rays up to 1018 eV appear to be isotropic, due to losing all their directional information in cosmic magnetic fields

  13. The knee, possible explanations • Galactic confinement • Particles down to the knee can be contained in our galaxy due to its magnetic field • below the knee, particles start to leak out of our galaxy • Maximum energy supplied by supernovae

  14. The ankle, a theory • Extragalactical component? • The spectrum could be the combination of a hard extragalactic spectrum and a steeper galactic spectrum

  15. Toes? The GZK cut-off (Greisen, Zatsepin and Kuzmin) • It is expected that the CMB becomes opaque for protons above 1019 eV • Thus the protons have a limited range James Cronin, 2007

  16. The problem • Fly`s Eye measured an Oh-My-God particle in 1991, with an energy of 3.2x1020 eV • The assumption is that the ankle has an extragalactic origin • Particles of that energy must originate very close to our galaxy ?

  17. A closer look at the toes • According to AGASA measurements there is no energy cut-off visible at all -> No Toes

  18. What's so great about these energies? • According to the GZK cut-off they have to originate very close to us • In addition we could actually pinpoint their sources!

  19. James Cronin, 2007

  20. James Cronin, 2007

  21. Pierre Auger Data James Cronin, 2007

  22. Pierre Auger Data

  23. Summary • We know do not really have any top-down models to explain the primary cosmic ray flux • We know a few bottom up approaches, but they party don’t make much sense, and are very limited in energy • We do have several explanations ready for the knee, the ankle and supposed toes, but the big picture does not make sense at the moment Michael Turner, 2007

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