Butsuri
Online ISSN : 2423-8872
Print ISSN : 0029-0181
ISSN-L : 0029-0181
Observation of Ultra-High Energy Cosmic Rays by Telescope Array(Current Topics)
Yoshiki TsunesadaShoichi OgioHiroyuki Sagawa
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2013 Volume 68 Issue 10 Pages 670-674

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Abstract

The results from the Telescope Array experiment after the first five year observation are reviewed. We measured the energy spectrum of ultra-high energy cosmic rays (UHECRs) by two different types of detectors, the surface detector array (SD) and the fluorescence detectors (FDs). The energy spectrum by TA is fully consistent with the result from the HiRes experiment in the energy scale and the shape. We found a steepening in the energy spectrum at 5.4 x 10^<19> eV, which is also consistent with the claim firstly reported by HiRes, and later by Auger. The analysis of longitudinal development of air showers in the atmosphere shows that the UHECR mass composition is dominated by protons around the steepening. This is a supporting evidence of the Greisen-Zatsepin-Kuzmin cutoff, which may be formed as a result of UHECR energy losses via interaction with the cosmic microwave background photons in the inter-galactic space. However, we found no statistically significant anisotropy in the arrival directions of the current TA data, which is expected if UHECRs from nearby sources are preferably observed in the energy region beyond the steepening. The interpretation of the results and the future prospects are discussed.

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© 2013 The Physical Society of Japan
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