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Session 9 - High-Energy Phenomena.
Display session, Monday, June 08
Atlas Ballroom,
Taking advantage of the dark skies near Dugway, Utah, the University of Utah's Fly's Eye Detector has been observing cosmic rays with energies above 3\times 10^16 eV by detecting light from the nitrogen fluorescence from the Extensive Air Showers produced by cosmic rays.
The detection of an event measured to have 3\times 10^20 eV raises the question of whether the cosmic ray spectrum continues above the energy where cosmic rays should lose energy to photopion production with the 3 K cosmic microwave background. This question will only be answered with detectors having a larger detection area (aperture). The University of Utah Cosmic Ray Group is currently constructing the High Resolution Fly's Eye Detector (HiRes) which will have at least an order of magnitude more aperture than the original Fly's Eye, enabling it to achieve higher statistics above 10^20 eV which will better characterize whether the spectrum continues past the photopion production threshold.
Evidence will be presented that the composition of cosmic rays from 10^17 eV to 10^19 eV changes from primarily heavy nuclei such as iron at 10^17 eV, to more of a light nuclei dominated composition at 10^19 eV. A change in composition through this energy range could be evidence that the sources of cosmic rays shifts from galactic at 10^17 eV to extragalactic at 10^19 eV. The extragalactic origin of cosmic rays beyond 10^19 eV is also supported by the isotropic distribution of these energetic cosmic rays.
About 20 km from Fly's Eye, the University of Tokyo's Telescope Array Detector has been detecting TeV gamma rays for the past couple of years. The biggest challenge for TeV gamma ray detection is to pick the gamma ray showers out of the cosmic ray shower noise, because at TeV energies only approximately 1/1000 shower events are gammas rather that hadrons. During 1997, a flare from the active galaxy nucleus of Markarian 501 was detected, which appears may have an periodic variation of about 12 days.
