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When searching for anisotropies in the arrival directions of Ultra High Energy Cosmic Rays, one must estimate the number of events expected in each direction of the sky in the case of a perfect isotropy. We present in this article a new method, developed for the Auger Observatory, based on a smooth estimate of the zenith angle distribution obtained from the data itself (which is essentially unchanged in the case of the presence of a large scale anisotropy pattern). We also study the sensitivity of several methods to detect large-scale anisotropies in the cosmic ray arrival direction distribution : Rayleigh analysis, dipole fitting and angular power spectrum estimation.
Anisotropy in the cosmic-ray arrival direction distribution has been well documented over a large energy range, but its origin remains largely a mystery. In the TeV to PeV energy range, the galactic magnetic field thoroughly scatters cosmic rays, but anisotropy at the part-per-mille level and smaller persists, potentially carrying information about nearby cosmic-ray accelerators and the galactic magnetic field. The IceCube Neutrino Observatory was the first detector to observe anisotropy at these energies in the Southern sky. This work uses 318 billion cosmic-ray induced muon events, collected between May 2009 and May 2015 from both the in-ice component of IceCube as well as the surface component, IceTop. The observed global anisotropy features large regions of relative excess and deficit, with amplitudes on the order of $10^{-3}$. While a decomposition of the arrival direction distribution into spherical harmonics shows that most of the power is contained in the low-multipole ($\ell \leq 4$) moments, higher-multipole components are found to be statistically significant down to an angular scale of less than $10^{\circ}$, approaching the angular resolution of the detector. Above 100\,TeV, a change in the topology of the arrival direction distribution is observed, and the anisotropy is characterized by a wide relative deficit whose amplitude increases with primary energy up to at least 5\,PeV, the highest energies currently accessible to IceCube with sufficient event statistics. No time dependence of the large- and small-scale structures is observed in the six-year period covered by this analysis within statistical and systematic uncertainties. Analysis of the energy spectrum and composition in the PeV energy range as a function of sky position is performed with IceTop data over a five-year period using a likelihood-based reconstruction. Both the energy spectrum and the composition distribution are found to be consistent with a single source population over declination bands. This work represents an early attempt at understanding the anisotropy through the study of the spectrum and composition. The high-statistics data set reveals more details on the properties of the anisotropy, potentially able to shed light on the various physical processes responsible for the complex angular structure and energy evolution.
The MINOS Near and Far Detectors are two large, functionally-identical, steel-scintillating sampling calorimeters located at depths of 220 mwe and 2100 mwe respectively. The detectors observe the muon component of hadronic showers produced from cosmic ray interactions with nuclei in the earth's atmosphere. From the arrival direction of these muons, the anisotropy in arrival direction of the cosmic ray primaries can be determined. The MINOS Near and Far Detector have observed anisotropy on the order of 0.1% at 1 and 11 TeV respectively. The amplitude and phase of the first harmonic at 1 TeV are 8.2 ± 1.7(stat.) x 10−4 and (8.9 ± 12.1(stat.)){sup o}, and at 11 TeV are 3.8 ± 0.5(stat.) x 10−4 and (27.2 ± 7.2(stat.)){sup o}.
The book reviews methods for the analysis of astronomical datasets, particularly emphasizing very large databases arising from both existing and forthcoming projects, as well as current large-scale computer simulation studies. Leading experts give overviews of cutting-edge methods applicable in the area of astronomical data mining.
This volume presents the current knowledge of magnetic fields in diffuse astrophysical media. Starting with an overview of 21st century instrumentation to observe astrophysical magnetic fields, the chapters cover observational techniques, origin of magnetic fields, magnetic turbulence, basic processes in magnetized fluids, the role of magnetic fields for cosmic rays, in the interstellar medium and for star formation. Written by a group of leading experts the book represents an excellent overview of the field. Nonspecialists will find sufficient background to enter the field and be able to appreciate the state of the art.