33ND INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013 THE ASTROPARTICLE PHYSICS CONFERENCE

Abstract: We have published the energy spectrum of ultra-high energy cosmic rays (UHECRs) for E> 1018.2 eV from our first 5-year observation with the surface detectors of Telescope Array. We found two breaking points in the power-law spectrum, a flattening known as the ”ankle ” or ”dip ” at E = 1018.66 eV, and a steepening at E = 1019.72 eV. The spectral features must be related to distribution of cosmic ray sources, injection spectrum, and energy loss processes during propagation in inter-galactic space. We constructed a model of source distribution of UHECRs and energy loss processes in the CMB photon field to calculate energy spectrum to be observed at the Earth. The model includes injection energy spectra at the sources, energy loss processes, and evolution of cosmic ray fluxes with red shift. We consider two cases that the UHECR sources are distributed uniformly, or same as galaxies in the local universe. We found that the spectrum can be well fitted with the model. We discuss to give a constraint to the model parameters, e.g. the injection power indices and cosmological evolution from the observed data