Measurement of energy spectrum of ultra-high energy cosmic rays

Ultra-high energy cosmic rays (UHECRs) are charged particles of energies above 1018 eV that originate outside the Galaxy. Because the flux of the UHECRs at Earth is very small, the only practical way of observing UHECRs is by measuring the extensive air showers produced by UHECRs in the atmosphere. This is done by using air fluorescence detectors and giant arrays of particle detectors on the ground. The PierreAuger Observatory (Auger) andTelescopeArray (TA) are two large cosmic ray experiments that use such techniques and cover 3000 and 700 km(2) areas on the ground, respectively. In this paper, we present the UHECR spectrum reported by TA, using an exposure of 6300 km(2) sr yr accumulated over 7 years of data taking, and the corresponding result of Auger, using 10 years of data with a total exposure exceeding 50 000 km(2) sr yr. We review the astrophysical interpretation of the two measurements, and discuss their systematic uncertainties.