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Opis: We present results of the measurement of large-scale anisotropies in the arrival directions of ultra–high-energy cosmic rays detected at the Pierre Auger Observatory during 19 yr of operation, prior to AugerPrime, the upgrade of the observatory. The 3D dipole amplitude and direction are reconstructed above 4 EeV in four energy bins. Besides the established dipolar anisotropy in right ascension above 8 EeV, the Fourier amplitude of the 8–16 EeV energy bin is now also above the 5σ discovery level. No time variation of the dipole moment above 8 EeV is found, setting an upper limit to the rate of change of such variations of 0.3% per year at the 95% confidence level. Additionally, the results for the angular power spectrum are shown, demonstrating no other statistically significant multipoles. The results for the equatorial dipole component down to 0.03 EeV are presented, using for the first time a data set obtained with a trigger that has been optimized for lower energies. Finally, model predictions are discussed and compared with observations, based on two source emission scenarios obtained in the combined fit of spectrum and composition above 0.6 EeV.
Ključne besede: ultra–high-energy cosmic rays, UHECRs, UHECR anisotropies, Pierre Auger Observatory, dipolar anisotropy in right ascension, Fourier amplitude analysis, angular power spectrum, equatorial dipole component, UHECR source emission scenarios
Objavljeno v RUNG: 26.11.2024; Ogledov: 203; Prenosov: 0
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Opis: The distribution of ultra-high-energy cosmic-ray arrival directions appears to be nearly isotropic except for a dipole moment of order 6×(E/10 EeV) per cent. Nonetheless, at the highest energies, as the number of possible candidate sources within the propagation horizon and the magnetic deflections both shrink, smaller-scale anisotropies might be expected to emerge. On the other hand, the flux suppression reduces the statistics available for searching for such anisotropies. In this work, we consider two different lists of candidate sources: a sample of nearby starburst galaxies and the 2MRS catalog tracing stellar mass within 250 Mpc. We combine surface-detector data collected at the Pierre Auger Observatory until 2020 and the Telescope Array until 2019, and use them to test models in which UHECRs comprise an isotropic background and a foreground originating from the candidate sources and randomly deflected by magnetic fields. The free parameters of these models are the energy threshold, the signal fraction, and the search angular scale. We find a correlation between the arrival directions of 11.8%+5.0%−3.1% of cosmic rays detected with E≥38 EeV by Auger or with E≳49 EeV by TA and the position of nearby starburst galaxies on a 15.5∘+5.3∘−3.2∘ angular scale, with a 4.2σ post-trial significance, as well as a weaker correlation with the overall galaxy distribution.
Ključne besede: Pierre Auger Observatory, Telescope Array, indirect detection, surface detection, ground array, ultra-high energy, cosmic rays, anisotropy, full-sky, starburst galaxies, source correlations, dipole
Objavljeno v RUNG: 04.10.2023; Ogledov: 1810; Prenosov: 5
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Opis: The sources of ultra-high-energy cosmic rays are still unknown, but assuming standard physics, they are expected to lie within a few hundred megaparsecs from us. Indeed, over cosmological distances cosmic rays lose energy to interactions with background photons, at a rate depending on their mass number and energy and properties of photonuclear interactions and photon backgrounds. The universe is not homogeneous at such scales, hence the distribution of the arrival directions of cosmic rays is expected to reflect the inhomogeneities in the distribution of galaxies; the shorter the energy loss lengths, the stronger the expected anisotropies. Galactic and intergalactic magnetic fields can blur and distort the picture, but the magnitudes of the largest-scale anisotropies, namely the dipole and quadrupole moments, are the most robust to their effects. Measuring them with no bias regardless of any higher-order multipoles is not possible except with full-sky coverage. In this work, we achieve this in three energy ranges (approximately 8-16 EeV, 16-32 EeV, and 32-∞ EeV) by combining surface-detector data collected at the Pierre Auger Observatory until 2020 and at the Telescope Array (TA) until 2019, before the completion of the upgrades of the arrays with new scintillator detectors. We find that the full-sky coverage achieved by combining Auger and TA data reduces the uncertainties on the north-south components of the dipole and quadrupole in half compared to Auger-only results.
Ključne besede: Pierre Auger Observatory, Telescope Array, indirect detection, surface detection, ultra-high energy, cosmic rays, anisotropy, large scale, fully sky coverage, dipole, quadropole
Objavljeno v RUNG: 29.09.2023; Ogledov: 1446; Prenosov: 5
Celotno besedilo (2,66 MB)
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