Measurement of the Anisotropy of Cosmic Ray Arrival Directions with IceCube

All of the TeV measurements were performed in the Northern hemisphere; so far, no such measurement has been performed covering the entire Southern hemisphere at median energies in the multi TeV region. With the deployment of the IceCube Neutrino Observatory at the South Pole, we have for the first time measured the anisotropy at TeV energies in the Southern sky. Ice-Cube is primarily a neutrino detector but it is sensitive to the muons produced in downward-going cosmic ray air showers. The observatory provides high-statistics measurements of cosmic rays with median energy of 20 TeV.

When completed in 2011, IceCube will comprise 5160 optical modules buried 1450 and 2450 meters below the surface of the polar ice sheet. The modules are physically connected to the surface by electronic umbilical lines, or “strings,” with 86 strings in total (Abbasi et al. 2009). In this paper, we use cosmic ray data recorded by the detector in its 22-strings configuration (IC22) between June 2007 and March 2008 to produce the cosmic ray skymap of the Southern sky in the TeV range.

During the IC22 physics run, cosmic ray events were observed at an average trigger rate of about 550 Hz. The arrival direction is determined by a likelihood based reconstruction which is seeded with a fast online estimate of the arrival direction (Ahrens et al. 2004). The likelihood based reconstruction is applied if twelve or more optical sensors on at least three strings were triggered by the event. A total of 5.2 × 10 9 events satisfied the above conditions at an average rate of ∼ 240 Hz. Further selection criteria were applied to the data to ensure good quality and stable runs. The final data set contains 4.3 × 10 9 events with a total livetime of 226 days, a median angular resolution of 3 • , and a median energy per cosmic ray of 20 TeV. The energy scale was determined with a standard cosmic ray simulation program, COR-SIKA (CORSIKA 2009), using the SIBYLL hadronic interaction model (Version 2.1) (Engel 1999) and the Poly-Gonato model for the composition and spectrum of the primary cosmic rays (Hörandel 2003).

To evaluate physical anisotropies in the cosmic ray data set, it is necessary to eliminate spurious effects which can mimic an anisotropy. These include local effects such as diurnal and seasonal variations of atmospheric conditions, asymmetries in the detector geometry, and nonuniform detector exposure to different regions of the sky. Fortunately, the location of IceCube at the South Pole is ideal to compensate for many effects that can impact cosmic ray detectors in the middle latitudes. At the South Pole, the Southern celestial sky is fully visible at any given time, providing complete and uniform coverage. While the seasonal variation in the cosmic ray event rate is on the order of ±10% (Tilav et al. 2010), these variations are sufficiently slow to have no effect on the anisotropy. Rapid atmospheric changes which can affect the rate are rare and can be identified from the data.

The remaining effects which must be accounted for in this analysis are an asymmetry in the IceCube detector response, and a non-uniformity in the time coverage of the data. The asymmetric response is due to the geometrical configuration of IceCube during the IC22 physics run (as shown in Figure 1); events arriving along the long axis of the detector were preferentially selected by the online filter and reconstruction due to the larger number of strings and modules triggered. In principle, the rotation of the Earth should average out the local asymmetry in the arrival directions each day, but gaps in the detector uptime and uneven run selection due to quality selection introduce non-uniformities into the time coverage of the data. These non-uniformities preclude the complete averaging, and translate into an artificial arrival direction asymmetry in equatorial coordinates. To correct for this detector-related asymmetry, each event from a given local azimuth bin i was weighted with the ratio n/n i , where n is the average number of events over the full range of local azimuths, and n i is the number of events in local azimuth bin i. Since the local azimuth distribution varies with zenith angle, the events were grouped into four zenith bands with approximately equal numbers of events per band. The weighting is applied within each band to remove the detector asymmetry.

To investigate the arrival direction distribution of the cosmic rays, we studied the relative intensity of the cosmic ray induced muon flux. The arrival direction distribut

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