The isotropy problem of Sub-ankle Ultra-high energy cosmic rays

Abstract

We study the time dependent propagation of sub-ankle ultra-high energy cosmic rays (UHECRs) originating from point-like Galactic sources. We show that drift in the Galactic magnetic field (GMF) may play an important role in the propagation of UHECRs and their measured anisotropy, particularly when the transport is anisotropic. To fully account for the discreteness of UHECR sources in space and time, a Monte Carlo method is used to randomly place sources in the Galaxy. The low anisotropy measured by Auger is not generally characteristic of the theoretical models, given that the sources are distributed in proportion to the star formation rate, but it can possibly be understood as a) intermittency effects due to the discrete nature of the sources or, with extreme parameters, b) a cancellation of drift current along a current sheet with the outward radial diffusive flux. We conclude that it is possible to interpret the Galactic sub-ankle CR flux as being due entirely to intermittent discrete Galactic sources distributed in proportion to star formation, but only with probability of roughly 35 percent, of which the spectrum is in accord with observations about 30 percent of the time. An alternative explanation for the low anisotropy may be that they are mostly extragalactic and/or heavy.

submitted to ApJ

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Cited by in corpus (8)

• Observation of a Large-scale Anisotropy in the Arrival Directions of Cosmic Rays above $8\times 10^{18}$ eV
• Cosmic Ray Models
• Indication of a Local "Fog" of Sub-Ankle UHECR
• Cosmic Rays around $10^{18}$eV: Implications of Contemporary Measurements on the Origin of the Ankle Feature
• Selected Topics in Cosmic Ray Physics
• Ultrahigh Energy Cosmic Rays: A Galactic Origin?
• Time variability of TeV cosmic ray sky map
• Particle Spectra and Mass Composition in the Ultra-High Energy Region in the Framework of the Galactic Origin of Cosmic Rays