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The Most Energetic Cosmic Rays

The Most Energetic Cosmic Rays. Joanna Bridge Astro 550 December 7, 2012. Roadmap. Introduction to cosmic rays The cosmic ray spectrum Detecting cosmic rays Ultra-high-energy cosmic rays The GZK cutoff The Pierre Auger Observatory Facilities Selected Results.

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The Most Energetic Cosmic Rays

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  1. The Most Energetic Cosmic Rays Joanna Bridge Astro 550 December 7, 2012

  2. Roadmap • Introduction to cosmic rays • The cosmic ray spectrum • Detecting cosmic rays • Ultra-high-energy cosmic rays • The GZK cutoff • The Pierre Auger Observatory • Facilities • Selected Results

  3. A brief history of the cosmic ray • 1909 - Theodore Wulf created an electrometer to measure ion production • Showed that ionization levels at the top of the Eiffel Tower were higher than on the ground • 1912 - Victor Hess ascended 5300 m in a balloon during a solar eclipse with 3 electrometers • Showed ionization levels 4x higher than the ground • Not solar in origin • 1936 - Hess won Nobel Prize Victor Hess in at the end of his balloon flight in 1912

  4. Brushing up on cosmic rays… • What are cosmic rays? • ~90% protons, ~9% -particles, ~1% electrons • The rest are heavier nuclei • Li, B, Be • The heavier nuclei are the result of cosmic ray spallation

  5. Brushing up on cosmic rays… Where do cosmic rays come from? ?

  6. A quick and dirty guide to the cosmic ray spectrum Follows a power law with index -2.7 until reaching the knee The ankle occurs at ~5x1018 eV The knee occurs at ~4x1015 eV Cosmic rays above ~1020 eV are a great mystery! (More on that later…)

  7. A slightly more complicated version of the same thing

  8. There are various ways to detect cosmic rays • The neutral component of cosmic rays (i.e. -rays) can be detected by instruments up to 100 GeV • Compton Gamma Ray Observatory • Fermi Space Telescope • Charged cosmic rays (i.e. protons) are detectable up to 100 TeV • Above 100 TeV, current instruments cannot provide significant detection, but the effects of the radiation can be detected

  9. Cherenkov Effect When a cosmic ray hits the atmosphere, it results in an energetic particle shower

  10. Cherenkov Radiation Particles that travel faster through a medium than the speed of light can in that medium (c/n) emit light Compare to a “sonic boom” produced by plane traveling faster than speed of sound Reed Research Reactor emitting Cherenkov radiation

  11. There are two ways that the Cherenkov Effect can be observed 1.) The shower of particles created by the incident cosmic ray (I.e. hadrons, muons and electrons) can be detected -With multiple detectors, the lateral cross-section of the shower can be determined and thus the initial energy of the cosmic ray 2.) Additionally, the Cherenkov light itself can also be measured

  12. Every presentation needs a good movie…

  13. …or two.

  14. So how about those ultra-high-energy (UHE) cosmic rays? • UHE cosmic rays are a bit of an enigma • Where do they come from? • How do they get to be so energetic? • UHE cosmic rays probe the farthest reaches of current physical theories • Quantum gravity • Special relativity

  15. The Greisen–Zatsepin–Kuzmin (GZK) limit is a theoretical upper bound on cosmic ray energy Extragalactic high energy cosmic rays can collide with CMB photons • Below ~1017 eV, this results in pair production • Above ~1017 eV, pion production results CMB + p  +  n + + CMB + p  +  p + 0 This process drains 20% of the cosmic ray’s energy (as opposed to 0.1% from pair production)

  16. The Greisen–Zatsepin–Kuzmin (GZK) limit is a theoretical upper bound on cosmic ray energy Pion production occurs until the cosmic ray energy drops below ~1017 eV, and standard pair production picks up again The mean free path of this interaction is such that UHE cosmic rays that travel more than 50 Mpc should never be observed on Earth This energy cutoff is ~6x1019 eV

  17. Cosmic rays have been detected with energies higher than the GZK cutoff • 1962 - First UHECR detected at >1x1020 eV (New Mexico) • 1991 - Highest energy UHECR: 3.2x1020 eV (Utah) v = 0.9999999999999999999999951c Some ~100 UHE cosmic rays have been observed in total If the GZK cutoff has indeed been violated, it would lead to one of the biggest unsolved questions in physics

  18. Pierre Auger Observatory • Located on the plain of Pampa Amarilla, near the town of Malargüe in Mendoza Province, Argentina • Collaboration among 17 institutions

  19. The Auger Observatory was built to detect UHECRs 2005 - First science grade data taken 2008 - Observatory was fully completed Consists of 1600 water Cherenkov surface detectors and 4 atmospheric fluorescence detectors Spread over 3000 km2, creating a detector the size of Rhode Island

  20. How the Auger Cherenkov surface detectors work • The water tank station detects a strength of signal that depends on its distance from the shower axis • The direction is calculated from the shower front arrival times in different detectors • Detectors are 1.5 km apart, each one an 11,000-liter tank filled with 12 tons of pure water

  21. The fluorescence detectors observe the light cone from the particle shower • Detectors consist of a mirror array and camera • The fluorescence detectors can measure cosmic ray showers in more detail than the giant array but can only observe fluorescence on dark nights

  22. The cosmic ray spectrum from Auger observatory The steepening (~1019.5 eV) is consistent with the GZK cutoff The ankle (~1018.5 eV) is possible a transition between CRs of galactic and extragalactic origin

  23. Early results showed a correlation between UHECRs and AGN • 19 out of 27 events correlated with AGN from the Véron-Cetty catalog for a 69% correlation

  24. More recent results throw this correlation into question • 29 out of 69 events correlated, decreasing the percentage to 38% • Compare this to 21% that would appear to be correlated by chance

  25. A surprising result! This plot shows how the shower maximum depends on the nature of the cosmic ray The more energetic the cosmic ray, the deeper the shower maximum occurs, with photons penetrating the most Photons Protons Iron Nuclei The surprise was that by measuring this “elongation rate”, iron was found to be the most common UHECR, not protons

  26. An aside: GRBs are not the (only) source of UHECRs • Neutrinos should be produced by interactions between UHECRs and the photon field of a GRB • Models indicate at least 8.4 neutrinos should be detected • A recent report from IceCube indicates that that exactly zero from the GRBs that went off during their data runs

  27. Summary • UHE cosmic rays are still largely a mystery • It is as yet unknown if GZK cutoff has been violated • The Pierre Auger Observatory has made headway on the origins and nature of UHECRs • Possibly AGN, but questionable • Further data will help answer these questions

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