Bright Cosmic-Ray Source as a Solution to Auger-TA Tensions

TL;DR

This paper proposes that a single nearby starburst galaxy source, such as M82, can explain the spectral and anisotropy differences observed between Auger and TA ultra-high-energy cosmic ray measurements, unifying their data.

Contribution

It introduces a model where a nearby cosmic-ray source accounts for spectral discrepancies and anisotropy, aligning with previous composition fits and hotspot observations.

Findings

A nearby source explains spectral differences between Auger and TA.

The source's position near M82 is consistent with observations.

The model reproduces the observed dipole shift without extra tuning.

Abstract

The ultra-high-energy cosmic ray (UHECR) spectra measured by the Pierre Auger Observatory (Auger) and the Telescope Array (TA) agree very well below $10^{19.5}$ eV but differ significantly at higher energies. We show that these differences can be explained by a single nearby source superimposed on a nearly isotropic background. Taking into account deflections in Galactic and extragalactic magnetic fields, such a source can account for the excess in the TA spectrum without producing excessive anisotropy. The required hard spectrum of the source and intermediate-mass composition are consistent with previous fits of the Auger-only spectrum and composition. This scenario offers several additional advantages: (i) the source produces a broad excess partially overlapping the TA hotspots, suggesting their possible explanation; (ii) without additional tuning, it reproduces the $\sim90^\circ$ shift in dipole direction observed between the Auger-only and combined Auger-TA analyses; and (iii) the best-fit position of the source lies near M82, the brightest nearby starburst galaxy, making it a plausible source of the UHECR.