IceCube astrophysical neutrinos without a spectral cutoff and $10^{15}$–$10^{17}$ eV cosmic gamma radiation

We present a range of unbroken power-law fits to the astrophysical-neutrino spectrum consistent with the most recent published IceCube data at the 68 % confidence level. Assuming that the neutrinos originate in decays of $\pi$-mesons, we estimate accompanying gamma-ray fluxes for various distributions of sources, taking propagation effects into account. We then briefly discuss existing experimental results constraining PeV to EeV diffuse gamma-ray flux and their systematic uncertainties. Several scenarios are marginally consistent both with the KASKADE and CASA-MIA upper limits at $10^{15}$–$10^{16}$ eV and with the EAS-MSU tentative detection at $\sim 10^{17}$ eV, given large systematic errors of the measurements. Future searches for the diffuse gamma-ray background at sub-PeV to sub-EeV energies just below present upper limits will give a crucial diagnostic tool for distinguishing between the Galactic and extragalactic models of the origin of the IceCube events.