Evidence of Intermediate-Scale Energy Spectrum Anisotropy of Cosmic Rays E$\geq$10$^{19.2}$ eV with the Telescope Array Surface Detector

TL;DR

This study reports evidence of intermediate-scale anisotropy in ultra-high energy cosmic rays above 10^19.2 eV, revealing directional variations in their arrival directions using 7 years of Telescope Array data.

Contribution

It provides the first evidence of intermediate-scale anisotropy in cosmic ray arrival directions at these energies, using a novel statistical approach and extensive data analysis.

Findings

Significant anisotropy detected at specific sky coordinates.

Deficit of events below 10^19.75 eV in the anisotropic region.

Global significance of anisotropy is 3.74 sigma.

Abstract

An intermediate-scale energy spectrum anisotropy has been found in the arrival directions of ultra-high energy cosmic rays of energies above $10^{19.2}$ eV in the northern hemisphere, using 7 years of data from the Telescope Array surface detector. 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$$16^m$, $45^{\circ}$. and has a deficit of events with energies $10^{19.2}$$\leq$$E$$<$$10^{19.75}$ eV and an excess for $E$$\geq$$10^{19.75}$ eV. The post-trial probability of this energy anisotropy, appearing by chance anywhere on an isotropic sky, is found by Monte Carlo simulation to be $9$$\times$$10^{-5}$ ($3.74$$\sigma_{global}$).