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

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.

Figures (6)

• Figure 1: TA TA_8sigma and Auger PierreAuger:2025hnw spectra shifted to the best-fit common energy scale in the energy range from $10^{18.75}$ eV to $10^{19.5}$ eV (highlighted with the gray shading).
• Figure 2: Significance of the residual TA–Auger spectral discrepancy as a function of source position $(l, b)$ plotted in Galactic coordinates. Different panels correspond to different levels of additional IGMF smearing, $\theta_\mathrm{19}^\mathrm{IGMF}$. In each skymap, the Galactic Center (GC) is shifted to the right for better visibility. The blue regions on the maps correspond to the positions of the source where the TA-Auger discrepancy is significantly reduced.
• Figure 3: Left panel: Spectral fit to the Auger and TA data, shown with dashed and solid red lines, respectively. The source contributions of C, Si, and Fe nuclei are indicated by yellow, black, and green lines, where solid lines show the source contribution to the TA spectrum and dashed lines --- to Auger spectrum. Right panel: Auger spectrum in the three declination bands closest to the TA field of view from Ref. PierreAuger:2025hnw, compared with the model predictions.
• Figure 4: Left panel: UHECR flux map above 40 EeV, averaged over top-hat windows of radius $\Psi = 25^\circ$ for the best-fit model without IGMF. Black dashed circles indicate the TA hotspots, while the red contour shows the region of the sky not visible to Auger. Middle panel: Li–Ma significance map of $25^\circ$-radius hotspots above 40 EeV in the TA field of view based on the flux map from the left panel. Right panel: same as the middle panel, but for the Auger field of view. All maps are shown in Galactic coordinates, centered on the Galactic Center (G.C.). The black star indicates the source position. The gray regions in the middle and right panels indicate the portions of the sky not visible to TA and Auger, respectively.
• Figure 5: Dipole direction at different energies. Black cross with uncertainty contours show the dipole direction from Auger-only data, while blue cross and contour shows the dipole direction from the full sky analysis TelescopeArray:2025yvu. Black square and blue dot indicate the dipole direction in the best-fit model (background with Auger-measured dipole plus source, zero IGMF), calculated using only the Auger field of view and the full sky, respectively. The black star marks the source position. The background in each panel shows the best-fit model flux map for the corresponding energy range.