Ultra-high-energy nuclei, photons, and magnetic fields

Combined recent data from cosmic-ray detectors and gamma-ray detectors have produced some surprising insights regarding the sources of ultrahigh-energy cosmic rays (UHECRs), magnetic fields inside and outside the Milky Way, and the universal photon backgrounds. The energy-dependent composition of UHECRs implies a non-negligible contribution of sources located in the Milky Way, such as past gamma-ray bursts that took place in our Galaxy. Extended halos of distant sources seen in the Fermi data imply that intergalactic magnetic fields have average strengths of the order of a femtogauss. Such relatively low magnetic fields imply that the protons from distant blazars generate a detectable flux of secondary gamma rays in their interactions with the photon background. A comparison with the data shows an excellent agreement of the secondary photons with the spectra of distant blazars observed by atmospheric Cherenkov telescopes.