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Abstract
Cosmic rays in the TeV to PeV energy range are believed to originate in our galaxy, possibly in local astrophysical accelerators such as supernova remnants. After escaping from their sources, cosmic rays propagate through the interstellar medium where they scatter off turbulences in the Galactic magnetic field. This scattering process efficiently isotropizes the trajectories of cosmic rays before their arrival at Earth. However, it is predicted that a dipolar anisotropy with per-mille amplitude or lower should subsist in their arrival directions.
Such an anisotropy was observed for the first time at TeV energies by detectors in the northern hemisphere, and its study revealed the presence of both large angular scale structure (usually interpreted as the aforementioned dipole produced by cosmic-ray diffusion) and anisotropy with smaller angular size and amplitude.
Large-scale anisotropy at TeV energies in the southern hemisphere was detected for the first time using data from IceCube, a cubic-kilometer neutrino detector that is sensitive to muons created in the interaction of cosmic rays with the atmosphere. The orientation of the large-scale component is consistent with that observed in the north.
In this work, results from three analyses are presented which expand the study of cosmic-ray anisotropy in the southern sky and are aimed at characterizing its evolution as a function of angular scale, energy, and time. Data from three cosmic-ray detectors are used: IceCube, its predecessor experiment AMANDA, and the IceTop air-shower array, all located at the South Pole.
Significant anisotropy is observed over a wide range of angular scales (from large-scale to few-degrees structure), and energies (from 20 TeV to 2 PeV.) The relative amplitude of the large-scale anisotropy is ∼ 10 -3, while smaller structures have amplitudes of the order of 10 -4. No significant variation is observed in the TeV anisotropy pattern over the 12-year period considered in this work.
These studies provide a complete picture of the cosmic-ray anisotropy in the southern sky at TeV and PeV energies. The coordinated study of the three main cosmic-ray observables (spectrum, composition, and anisotropy) will provide information about the origin of these particles and the environment through which they propagate.
