Restrictions on cosmogenic neutrinos and UHECR from Fermi 3 years data

Abstract

Ultra-high energy cosmic ray protons accelerated in astrophysical objects produce secondary electromagnetic cascades during propagation in the cosmic microwave and infrared backgrounds. Those cascades contribute to the GeV-TeV diffuse photon flux, measured by Fermi LAT experiment. Recent studies of 3 years of Fermi LAT data have shown that diffuse gamma-ray background at E > 10GeV is about factor of 2 smaller then original one year data. This affects both models of UHECR and secondary cosmogenic neutrino fluxes. We show the allowed range of cosmogenic neutrino fluxes scanning over unknown UHECR parameters such as injected proton maximum energy and power law index, evolution of sources, systematic shift of UHECR energy scale. We consider three evolution models in which the UHECR sources are assumed to have the same evolution of either the star formation rate (SFR), or the gamma-ray burst (GRB) rate, or the active galactic nuclei (AGN) rate in the Universe and found that the last two are disfavored (and in the dip model rejected) by the new VHE gamma-ray background. We show that the largest fluxes predicted in the dip model would be detectable by IceCube in about 10 years of observation and are within the reach of a few years of observation with the ARA project. In the incomplete UHECR model in which protons are assumed to dominate only above 1019 eV, the cosmogenic neutrino fluxes could be a factor of 2 or 3 larger.