The Extragalactic Ultra-high-energy Cosmic-Ray Dipole

Noemie Globus, Tsvi Piran

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22 Scopus citations

Abstract

We explore the possibility that the recently detected dipole anisotropy in the arrival directions of >8 EeV ultra-high-energy cosmic-rays (UHECRs) arises due to the large-scale structure. We assume that the cosmic-ray sources follow the matter distribution and calculate the flux-weighted UHECRs' rms dipole amplitude taking into account the diffusive transport in the intergalactic magnetic field (IGMF). We find that the flux-weighted rms dipole amplitude is ∼8% before entering the Galaxy. The amplitude in the [4-8] EeV is only slightly lower ∼5%. The required IGMF is of the order of 5-30 nG, and the UHECR sources must be relatively nearby, within ∼300 Mpc. The absence of a statistically significant signal in the lower-energy bin can be explained if the same nuclei specie dominates the composition in both energy bins and diffusion in the Galactic magnetic field reduces the dipole of these lower-rigidity particles. Photodisintegration of higher-energy UHECRs could also reduce somewhat the lower-energy dipole.

Original languageEnglish
Article numberL25
JournalAstrophysical Journal Letters
Volume850
Issue number2
DOIs
StatePublished - 1 Dec 2017

Bibliographical note

Publisher Copyright:
© 2017. The American Astronomical Society. All rights reserved.

Keywords

  • cosmic rays

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