Observation of Declination Dependence in the Cosmic Ray Energy Spectrum

TL;DR

This paper presents evidence of a significant difference in the ultrahigh energy cosmic ray spectrum between the northern and southern skies, suggesting potential anisotropy in cosmic ray sources or propagation.

Contribution

It provides the first combined analysis of data from the Telescope Array and Pierre Auger Observatory, validating the observed declination dependence of the cosmic ray spectrum.

Findings

8σ significance of spectrum difference between hemispheres

Validation of measurement methodology through overlapping sky region

Only 1.8σ difference after removing anisotropic regions

Abstract

We report on an observation of the difference between northern and southern skies of the ultrahigh energy cosmic ray energy spectrum with a significance of ${\sim}8\sigma$. We use measurements from the two largest experiments$\unicode{x2014}$the Telescope Array observing the northern hemisphere and the Pierre Auger Observatory viewing the southern hemisphere. Since the comparison of two measurements from different observatories introduces the issue of possible systematic differences between detectors and analyses, we validate the methodology of the comparison by examining the region of the sky where the apertures of the two observatories overlap. Although the spectra differ in this region, we find that there is only a $1.8\sigma$ difference between the spectrum measurements when anisotropic regions are removed and a fiducial cut in the aperture is applied.