Pinning down the cosmic ray source mechanism with new IceCube data

Abstract

Very recently the IceCube Collaboration has reported an observation of 28 neutrino candidates with energies between 50 TeV and 2 PeV, constituting a $4.1\ensuremath{\sigma}$ excess compared to the atmospheric background. In this article we investigate the compatibility between the data and a hypothesized unbroken power-law neutrino spectrum for various values of spectral index $\mathrm{\ensuremath{\Gamma}}\ensuremath{\ge}2$. We show that $\mathrm{\ensuremath{\Gamma}}\ensuremath{\sim}2.3$ is consistent at the $\ensuremath{\sim}1.5\ensuremath{\sigma}$ level with the observed events up to 2 PeV and to the null observation of events at higher energies. We then assume that the sources of this unbroken spectrum are Galactic, and deduce (i) an energy-transfer fraction from parent protons to pions (finding ${\ensuremath{\epsilon}}_{{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}}$ and ${\ensuremath{\epsilon}}_{\ensuremath{\pi}}$), and (ii) a way of discriminating among models which have been put forth to explain the ``knee'' and ``ankle'' features of the cosmic ray spectrum. Future IceCube data will test the unbroken power-law hypothesis and provide a multimessenger approach to explaining features of the cosmic ray spectrum, including the transition from Galactic to extragalactic dominance.