Testing the hadronuclear origin of PeV neutrinos observed with IceCube

We consider implications of the IceCube signal for hadronuclear (pp) scenarios of neutrino sources such as galaxy clusters/groups and star-forming galaxies. Since the observed neutrino flux is comparable to the diffuse gamma-ray background flux obtained by Fermi, we place new, strong upper limits on the source spectral index, Gamma <= 2.1-2.2. In addition, the new IceCube data imply that these sources contribute at least 30%-40% of the diffuse gamma-ray background in the 100 GeV range and even similar to 100% for softer spectra. Our results, which are insensitive to details of the pp source models, are one of the first strong examples of the multimessenger approach combining the measured neutrino and gamma-ray fluxes. The pp origin of the IceCube signal can further be tested by constraining Gamma with sub-PeV neutrino observations, by unveiling the sub-TeV diffuse gamma-ray background and by observing such pp sources with TeV gamma-ray detectors. We also discuss specific pp source models with a multi-PeV neutrino break/cutoff, which are consistent with the current IceCube data.