The UHECR dipole and quadrupole in the latest data from the original Auger and TA surface detectors

Abstract

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--$\infty$ 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.

proceedings of the 37th International Cosmic Ray Conference (ICRC 2021), 12-23 July 2021, Berlin, Germany

References in corpus (3)

• Observation of a Large-scale Anisotropy in the Arrival Directions of Cosmic Rays above $8\times 10^{18}$ eV
• Measurement of the cosmic-ray energy spectrum above $2.5\times 10^{18}$ eV using the Pierre Auger Observatory
• Search for Large-scale Anisotropy on Arrival Directions of Ultra-high-energy Cosmic Rays Observed with the Telescope Array Experiment

Cited by in corpus (5)

• Ultra-High-Energy Cosmic Rays: The Intersection of the Cosmic and Energy Frontiers
• Near-future discovery of point sources of ultra-high-energy neutrinos
• Anisotropies of Diffusive Ultra-high Energy Cosmic Rays in $f(R)$ Gravity Theory
• Modelling cross-correlations of ultra-high-energy cosmic rays and galaxies
• Explaining the UHECR spectrum, composition and large-scale anisotropies with radio galaxies