Evidence of Intermediate-scale Energy Spectrum Anisotropy of Cosmic Rays E ≥ 10 19.2 eV with the Telescope Array Surface Detector

Abstract

Evidence for an intermediate-scale energy spectrum anisotropy has been found in the arrival directions of ultra-high energy cosmic rays for energies greater than 10 19.2 eV in the northern hemisphere using 7 years of Telescope Array surface detector data. A relative energy distribution test is done comparing events inside oversampled spherical caps of equal exposure, to those outside, using the Poisson likelihood ratio. The center of maximum significance is at 9 h 16 m , 45°, and has a deficit of events with energies 10 19.2  ≤  E  < 10 19.75 eV and an excess for E  ≥ 10 19.75 eV. The post-trial probability of this energy anisotropy, appearing by chance anywhere on an isotropic sky, is found by Monte Carlo simulation to be 9 × 10 −5 (3.74 σ global ).