Pinning down the cosmic ray source mechanism with new IceCube data

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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.

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