Measurement of the Anisotropy of Cosmic Ray Arrival Directions with IceCube

We report the first observation of an anisotropy in the arrival direction of cosmic rays with energies in the multi-TeV region in theSouthern sky using data from the IceCube detector. Between 2007 June and 2008 March, the partially deployed IceCube detector was operated in a configuration with 1320 digital optical sensors distributed over22 strings at depths between 1450 and 2450 m inside the Antarctic ice. IceCube is a neutrino detector, but the data are dominated by a large background of cosmic-ray muons. Therefore, the background data aresuitable for high-statistics studies of cosmic rays in the southern sky. The data include 4.3 billion muons produced by downward-going cosmic-ray interactions in the atmosphere; these events were reconstructed with a median angular resolution of 3° and a median energy of ∼20 TeV. Their arrival direction distribution exhibits an anisotropy in right ascension with a first-harmonic amplitude of (6.4±0.2 stat.±0.8 syst.) × 10-4. © 2010 The American Astronomical Society.