Observation of Anisotropy in the Arrival Direction Distribution of Cosmic Rays above TeV Energies with Icecube

TL;DR

This paper reports the first observation of energy-dependent anisotropy in cosmic ray arrival directions in the southern hemisphere using IceCube data, revealing significant large- and small-scale features at TeV energies.

Contribution

It presents the first measurement of energy-dependent cosmic ray anisotropy in the southern hemisphere up to hundreds of TeV using IceCube's large dataset.

Findings

Large-scale anisotropy present at 20 TeV disappears at 400 TeV.

Localized excess and deficit regions observed between 10° to 30° scales.

Anisotropy features are statistically significant but their origins are unknown.

Abstract

Between May 2009 and May 2010, the IceCube neutrino detector recorded 32 billion of atmospheric muons generated in air showers produced by cosmic rays in the TeV energy range. With such high statistics sample it is possible to observe, for the first time in the southern hemisphere, an energy dependence in the Galactic cosmic ray anisotropy up to a few hundred TeV. This study shows that the same large-scale anisotropy observed at median energies around 20 TeV is not present at 400 TeV; the anisotropy observed at 400 TeV shows substantial differences with respect to that at lower energy. In addition to the large-scale features observed at 20 TeV in the form of strong dipole and quadrupole moments, the data include several localized regions of excess and deficit on scales between 10{\degree} to 30{\degree}. The features observed at both large and small scale are statistically significant, but their origin is currently unknown.