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Abstract: An update of the measurements of large-scale anisotropies in the arrival directions of ultra high-energy cosmic rays detected at the Pierre Auger Observatory is presented. The established dipolar anisotropy in right ascension has now reached a significance of 6.8σ when considering all energies above 8 EeV and 5.7 σ when only considering energies between 8 and 16 EeV. The 3D dipole amplitude and direction are reconstructed in four different energy bins above 4 EeV. At energies above 8 EeV it points more than 100∘ away from the Galactic centre, providing evidence that the anisotropy observed is of extragalactic origin. An analysis allowing for both dipolar and quadrupolar anisotropies finds qualitatively similar dipole components and no significant quadrupole components. The results for the angular power spectrum are shown, demonstrating that no other statistically significant multipoles are present. The equatorial dipole components are presented down to 0.03 EeV using a trigger which has been optimized for low energies. We find no significant departures from isotropic expectations below 8 EeV, although below 2 EeV the phases appear to be consistently aligned with the right ascension of the Galactic centre. Finally, model predictions based on source emission scenarios obtained in the combined fit of spectrum and composition data above 0.6 EeV are discussed and compared with observations.
Keywords: ultra-high-energy cosmic rays, Pierre Auger Observatory, large scale anisotropies
Published in RUNG: 24.03.2025; Views: 68; Downloads: 0
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Abstract: The large-scale distribution of arrival directions of high-energy cosmic rays is a key observable in attempts to understanding their origin. The dipole and quadrupole moments are of special interest in revealing potential anisotropies. An unambiguous measurement of these moments as well as of the full set of spherical harmonic coefficients requires full-sky coverage. This can be achieved by combining data from observatories located in both the northern and southern hemispheres. To this end, a joint analysis using data recorded at the Pierre Auger Observatory and the Telescope Array above 10[sup]19 eV has been performed. Thanks to the full-sky coverage, the measurement of the dipole moment reported in this study does not rely on any assumption on the underlying flux of cosmic rays. As well, the resolution on the quadrupole and higher order moments is the best ever obtained. The resulting multipolar expansion of the flux of cosmic rays allows a series of anisotropy searches to be performed, and in particular to report on the first angular power spectrum of cosmic rays. This allows a comprehensive description of the angular distribution of cosmic rays above 10[sup]19 eV.
Keywords: Pierre Auger Observatory, Telescope Array, high-energy cosmic rays, large-scale anisotropies, angular power spectrum
Published in RUNG: 08.03.2016; Views: 5741; Downloads: 193
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