American Astronomical Society, 192nd AAS Meeting

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Research Title: 
The Highest Energy Cosmic Rays and Gamma Rays
Research Abstract: 

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 3x 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 3x 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.