Testing the hadronuclear origin of PeV neutrinos observed with IceCube

Abstract

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 $\ensuremath{\gamma}$-ray background flux obtained by Fermi, we place new, strong upper limits on the source spectral index, $\ensuremath{\Gamma}\ensuremath{\lesssim}2.1--2.2$. In addition, the new IceCube data imply that these sources contribute at least 30%--40% of the diffuse $\ensuremath{\gamma}$-ray background in the 100 GeV range and even $\ensuremath{\sim}100%$ 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 $\ensuremath{\gamma}$-ray fluxes. The $pp$ origin of the IceCube signal can further be tested by constraining $\ensuremath{\Gamma}$ with sub-PeV neutrino observations, by unveiling the sub-TeV diffuse $\ensuremath{\gamma}$-ray background and by observing such $pp$ sources with TeV $\ensuremath{\gamma}$-ray detectors. We also discuss specific $pp$ source models with a multi-PeV neutrino break/cutoff, which are consistent with the current IceCube data.