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Register a new userObservation of a Large-scale Anisotropy in the Arrival Directions of Cosmic Rays above $8 times 10^{18}$ eV
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Cosmic rays are atomic nuclei arriving from outer space that reach the highest energies observed in nature. Clues to their origin come from studying the distribution of their arrival directions. Using $3 times 10^4$ cosmic rays above $8 times 10^{18}$ electron volts, recorded with the Pierre Auger Observatory from a total exposure of 76,800 square kilometers steradian year, we report an anisotropy in the arrival directions. The anisotropy, detected at more than the 5.2$sigma$ level of significance, can be described by a dipole with an amplitude of $6.5_{-0.9}^{+1.3}$% towards right ascension $alpha_{d} = 100 pm 10$ degrees and declination $delta_{d} = -24_{-13}^{+12}$ degrees. That direction indicates an extragalactic origin for these ultra-high energy particles.
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Results are presented that were obtained by analysing the arrival directions of E0 > 8x10**18 eV primary cosmic rays recorded at the Yakutsk array over the period between 1974 and 2003 and at the SUGAR array (Australia). The greatest primary cosmic ray flux is shown to arrive from the region of visible intersection of the planes of the Galaxy and the Supergalaxy (local supercluster of galaxies) at a galactic longitude of about 137 degres. On a global scale, the lowest temperature of the cosmic microwave background is typical of this region.
Anisotropy in the arrival directions of cosmic rays with energies above 10$^{17}$eV is studied using data from the Akeno 20 km$^2$ array and the Akeno Giant Air Shower Array (AGASA), using a total of about 117,000 showers observed during 11 years. In the first harmonic analysis, we have found strong anisotropy of $sim$ 4% around 10$^{18}$eV, corresponding to a chance probability of 0.2%. With two dimensional analysis in right ascension and declination, this anisotropy is interpreted as an excess of showers near the directions of the Galactic Center and the Cygnus region.
Spherical harmonic moments are well-suited for capturing anisotropy at any scale in the flux of cosmic rays. An unambiguous measurement 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 Telescope Array and the Pierre Auger Observatory above $10^{19}$ eV is presented in this work. The resulting multipolar expansion of the flux of cosmic rays allows us to perform a series of anisotropy searches, and in particular to report on the angular power spectrum of cosmic rays above $10^{19}$ eV. No significant deviation from isotropic expectations is found throughout the analyses performed. Upper limits on the amplitudes of the dipole and quadrupole moments are derived as a function of the direction in the sky, varying between 7% and 13% for the dipole and between 7% and 10% for a symmetric quadrupole.
We report a measurement of the energy spectrum of cosmic rays above $2.5{times} 10^{18}$ eV based on $215,030$ events. New results are presented: at about $1.3{times} 10^{19}$ eV, the spectral index changes from $2.51 pm 0.03 textrm{ (stat.)} pm 0.05 textrm{ (sys.)}$ to $3.05 pm 0.05 textrm{ (stat.)}pm 0.10textrm{ (sys.)}$, evolving to $5.1pm0.3textrm{ (stat.)} pm 0.1textrm{ (sys.)}$ beyond $5{times} 10^{19}$ eV, while no significant dependence of spectral features on the declination is seen in the accessible range. These features of the spectrum can be reproduced in models with energy-dependent mass composition. The energy density in cosmic rays above $5{times} 10^{18}$ eV is $(5.66 pm 0.03 textrm{ (stat.)} pm 1.40 textrm{ (sys.)} ) {times} 10^{53}~$erg Mpc$^{-3}$.
The amplitude and phase of the cosmic ray anisotropy are well established experimentally between 10^{11} eV and 10^{14} eV. The study of their evolution into the energy region 10^{14}-10^{16} eV can provide a significant tool for the understanding of the steepening (knee) of the primary spectrum. In this letter we extend the EAS-TOP measurement performed at E_0 around 10^{14} eV, to higher energies by using the full data set (8 years of data taking). Results derived at about 10^{14} and 4x10^{14} eV are compared and discussed. Hints of increasing amplitude and change of phase above 10^{14} eV are reported. The significance of the observation for the understanding of cosmic ray propagation is discussed.
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