Generation of 1015–1017 eV photons by ultrahigh-energy cosmic rays in the Galactic magnetic field

It is shown that the expected dip in the diffuse photon spectrum above the threshold of e+e− pair production, i.e., at energies 1015–1017 eV, may be absent due to the synchrotron radiation by the electron component of the extragalactic ultrahigh-energy cosmic rays (UHE CRs) in the Galactic magnetic field. The mechanism we propose requires small (<2×10−12 G) extragalactic magnetic fields and a large photon fraction in the UHE CRs. For a typical photon flux expected in top-down scenarios of UHE CRs, the predicted flux in the region of the dip is close to the existing experimental limit. The sensitivity of our mechanism to the extragalactic magnetic field may be used to improve existing bounds on the latter by two orders of magnitude.